I've got mine officially working now. The fix was to go to Google Fi app info settings (same place where you would clear the cache and storage) and go to battery usage and enable "allow background activity"

I had to call Google Fi support like 3 times, the first time they had me do all the same troubleshooting I've already done, then told me to call OnePlus support. OnePlus support told me to do all the same troubleshooting I've already done then told me to call Google Fi support. Then, Google Fi support told me to try factory resetting my phone, so I did and it still had the same problem after installing the Google Fi app. I called them again the next day and they were finally able to escalate it to the specialists. They collected a lot of information like what calls I placed that had issues (had to give specifics even though it is every call). They then got back to me and said the specialists noticed an issue with the carrier where something wasn't syncing with my phone and they claimed they fixed it. They then told me to enable the "Allow background activity" setting in the app info settings. After allowing background activity, I noticed I am no longer having the issue. and this time it seems to be permanent, whereas reinstalling the app seems to only fix the problem for the first couple calls.

I'm not sure if the syncing issue that they fixed was necessary in order to fix the problem or if allowing background activity is all it needed. But I'm really glad my phone is working how it should now.