Maybe hidden?
App Store > Tap your avatar > Tap your name. Scroll down, tap "Hidden Purchases".
Have these apps been getting updates? If not, can see, maybe, that running into a situation where the app is in some sort of abandoned status (eg. not fully pulled from the store) allowing for downloads but dropped from the purchased list. Quick scan of my purchased list shows a few apps that no longer have a cloud icon next to it under "Not on my". And digging into the full purchase history from the early days of App Store (App Store > avatar > name > Purchase History) shows some apps purchased that do not show up in the purchase or hidden list.
Seem to recall a story from a number of years ago where Apple was pulling app listings and pulling those apps from purchased list as well. Think it was tied to Apple's cleanup of the store. Kinda tied into, among other things, abandonware.
Nope, not hidden unfortunately. I do have a number of hidden apps, but the ones I am speaking of are not hidden (well, they don't show up on the list at all, so I don't even have the option to hide them-- ha).
One example is the Apollo app for Reddit. I have it on my iPhone and iPad, it's not hidden, and I receive regular updates for it. But, if I go into my purchased applications, it isn't listed there.
This has only been a minor inconvenience up until now. Whenever I get a new iPhone or iPad, I set it up as new (no backup) and then manually install my apps. Given many of them aren't on the Purchased list, I have resorted to taking screenshots of my home screen(s) when updating devices.
EDIT: What's odd is that a handful of these impacted apps are are paid apps. If I get a new device and go to install the app, I search for it on the App Store, and it will show the price instead of the cloud download symbol that you'd expect to see. I then have to "Purchase" the app instead-- but when I do, it says I have already purchased it and so it is restoring my purchase. So somewhere in the Apple system, it knows I've purchased these apps.