How do I manage multiple Apple IDs in Family Sharing for app access?

I have had iPhones for 20 years. Until a few years ago, I had to use one Apple account to use iTunes and download apps, and a separate account for iCloud. Most of my apps were downloaded using the Apple account associated with iTunes. In the last few years Apple have changed their minds and now say you should have only one account for all services. That meant the old apps would be orphaned, but a clever solution was found -- add your other identity as a "family share." I did so, and all my apps worked perfectly. Until today. Now, all of the dozens of apps I downloaded under the "iTunes identity" are failing to launch. ALL OF THEM. I deleted and re-downloaded two apps (using the newer "iCloud identity") and they work perfectly, so the problem is not with the apps.

The problem is that I don't want the hours-long job of deleting and re-downloading apps and placing them in their old folders. Nor do I want to pay again for a paid app purchased under the "iTunes identity." I did exactly as Apple asked for several years, maintaining two "Apple IDs," and I don't see why I should be punished for being a longtime customer.

I'm using an iPhone 12 mini running OS 18.5, and all apps have been updated. Phone capacity is 256GB, of which 89GB is available.

[Re-Titled by Moderator]

Original Title: Shared apps don't launch