I had the same thing happen to me last night. Woke the iPad after charging - when it shouldn't have been low enough to restart anyway - to find that the default widgets had reappeared. I deleted them again, tried to unscramble the mess it had made of my apps, and found that several of them weren't showing up at all. Settings showed that they still existed, but they weren't on any home page, and searching from the app library page didn't find them.
So I hard rebooted, the widgets reappeared again, the missing apps reappeared, and a completely different set disappeared. On this occasion that included things like Reminders and Weather, so I was able to launch them through the widgets. Though the app library search still told me they didn't exist.
This time I didn't delete the widgets immediately, but dragged them to a page on their own at the end. Rearranged everything else and checked what was missing. Took screenshots. Went to delete the widgets, at which point the iPad crashed, and when it came back up I still had the widgets (at the end where I'd put them) but my missing apps had reappeared. And this time when I deleted the widgets they stayed gone. I've rebooted several more times since. Fingers crossed, but it appears to be sticking though restarts now.
This is a week old M1 iPad Pro 11", which was upgraded to 16.3.1 before I did the original widget deletion. I'm not seeing it on my iPad Mini, where the widgets were deleted at a much earlier version. Not sure if that's a factor or if it's just completely random.