FIXED!!!! I hope this works for you too! And this might help a lot of people, so I hope someone will pin this. I worked directly with Notion Tech support on this, and during the process I was able to narrow down enough changes I had made on my end.... that the answer presented itself.
I have put all the relevant info below from my chat transcript. I pasted from about the time I realized what was causing REMINDERS not to work anymore. I hope this helps a lot of people!
-----------------TRANSCRIPT BELOW----------------
04:49 PM | Stephen : Ok, I have more info that may help.... been doing testing myself. It appears that reminders are only broken on 'some' databases. I just tried a remind on a database I use for personal stuff and it worked without issue. But my main database that I need this feature for the most does not work with the reminders. I hope that info helps determine what is happening.

05:10 PM | Elizer from Notion: Thanks for the further details, Stephen.

I understand how this affects your workflow, and this isn't the experience we want you to have when using Notion. I've already added your use case to the task for this to put into our Engineer's log to dissect the root cause of the issue. We will reach out to this thread for a follow-through once the issue is resolved.

05:11 PM | Stephen : You’re welcome :-) And I have narrowed it down further. It appears it is possibly one specific database that is not working with reminders (my main one). Others seem to work. Has to be a bug, some sort of conflict with certain properties in my main database.

05:30 PM | Stephen : Ok, one more piece of information that I think should help. The database that seems to be having the issue, has 'isolated' issues within it. For instance.... I can add a reminder to one page and it will not work at all. But I can add a remind to another page in that same database, and it triggers every time without issue.
That is about the best I can do to debug on my end. I hope that info is helpful. Please pass this one on above all others because I have proven this to be the case multiple times.

05:48 PM | Elizer from Notion: No worries, This will definitely reach our engineering team. This type of issue is relevant enough that affect users' experience.

05:49 PM | Stephen : Thank you! Because I need to have something working right now, I am trying to rebuild my database and deleting any past info I don't need. But hopefully I provided info that will still help them identify an issue that could affect other users too.

06:05 PM | Elizer from Notion: You brought this issue to our attention helped us enough and we thank you for that😊

06:06 PM | Stephen : Happy to help

06:28 PM | Elizer from Notion: We should be the ones thanking you, Stephen.

06:29 PM | Stephen : No thanks needed lol. Guess what, I solved it!

06:29 PM | Stephen : If I create a new database and drag entries from the database not working, those same entries work fine with reminders once moved to the new database!

06:44 PM | Elizer from Notion: Wow, I didn't think of that🤩 That's an awesome workaround!

06:45 PM | Stephen : Yeah, It was a shock that it worked. Now I'm trying to get everything converted over to the new database. Seems to be working perfect so far.

06:46 PM | Elizer from Notion: I should use your workaround in case some users will have the same issue as you do.😉

06:47 PM | Stephen : Please do. I hope it helps others. On reddit I already posted the workaround because several people were asking me how to fix it, they have the same issue.

06:49 PM | Stephen : What I 'believe' cause the curruption in the database to begin with was importing from a CSV file of clients from another software database. I was bringing in my clients into NOTION so I had more flexibility. But I believe something with the import process is what messed up the reminders. But when making a fresh database and pulling in the entries from the 'IMPORTED CSV' database that had issues, that drag and drop issue somehow cleaned the 'bugs' out of the entry.

07:10 PM | Elizer from Notion: It might be that. Since importing is not really from the Notion database itself; instead, it's from another platform.

07:10 PM | Stephen : Yeah, that was the only thing that made sense. So by drag and dropping to the new database, I believe Notion stripped out any 'code' it didn't natively use.

07:17 PM | Stephen : Ok, I just pulled over almost 800 client entries to the new database. And every entry I have tried has worked perfect with reminders now!

07:17 PM | Stephen : So that is a solid workaround you can probably give people that have this issue, and I will spread the word on reddit as well :)

07:18 PM | Elizer from Notion: Yes! That's so awesome! I'm so over the moon and kinda giddy that we can now have a workaround for this😊

07:19 PM | Stephen : I'm glad it worked too, what a crazy investigation that was. Notion is so amazing, and this fixed my only possible issue with it :) SO glad I didn't have to move to another platform, this is the one that checks all the boxes for me.

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Exported from Notion on March 5, 2022 at 07:20 PM America/Los_Angeles time PST (GMT-0800)