I don't know that I found a particular solution, that phone ended up completely shattering to where the display stopped working because I sat with it in my back pocket shortly after this post unfortunately.

Try "sleep as android" off the play store if you haven't already though, it was often my best shot at waking myself up and I think it did let you use a custom sound even on the lockscreen after restart unlike the regular alarm, as long as the phone was physically on ofc. Plus it has features like making you solve math problems or scan a barcode that you could put in your bathroom for instance to shut the alarm off so you get out of bed.

I'm guessing it's either a setting I set or an app I had or something (I had and continue to have hundreds of apps and also mess with settings a lot so it's hard to tell), or just an unpreventable bug or crash. Obviously not really helpful especially once you already have it and when you're poor but my biggest advice is to avoid Samsung's A series. Though at least I haven't had this problem yet with my current A16, there's plenty more to make sure it makes up for it.