Unfortunately, this happens all the time. App developers come and go, and apps are abandoned even when the developers are still in business. It's why you should not create anything on an iPad unless you are willing to lose it overnight without warning.
Happens with other platforms as well. I’ve lost content in the past on the Mac, and even mores so on older platforms. I admit the volume of small developers, and how easy it is to chose a less than great app (mostly out of app research fatigue) makes this more common on iOS. It’s also a common issue for older (pre Files app) apps, since they have far fewer options for storing and managing files.
That said, here is my approach to chooseing a productivity or creation app on any platform; providing you consider the content valuable.
Three things to look for, at least two of which should apply to your choice.,
1) Make sure the app is from a reliable developer with a long history of support and, well survival.
2) The app saves it’s files in a standard format that can be opened and edited by other apps. The more others, the better.
3) The app can export it’s files in other formats and to other (non-iPad) location
Other tips.
Keep an eye on how frequently your important apps get updated. No updates in a year is worrisome. A developer not updating their app soon after a major iOS update is a sign of neglect. If an app has not been updated for two years or more, and it does something you need, then you’d better start looking for a replacement, soon.
Having more than one app that does the same job, even if you don’t like it as much, is a good idea. Best if it works with the same file format, or can at least export/import the main app’s files. This can be a cheap, limited app. It’s just a safety net.
Make sure your important apps are all ready for a new version of iOS before updating. This requires a bit of patience and due-diligence. Professionals are often late adopters for this reason.
Do I always do these things? Well no. I often don’t realize that some files are important to me until they are gone. When that happens I bang my my head on a wall a few times, then go about doing what I can to salvage or recreate that work. I do pay close attention to all my main productive apps, which to be fair are mostly Apple’s apps, with Microsoft and Google thrown in when they can’t be avoided.