If Face ID unlock fails multiple times, the app offers an option to “Enter Password”, which leads to another set of Face ID attempts. If for whatever reason you can’t (or it’s inconvenient to) use Face ID, you’re essentially stuck.
Most apps, including the OS itself, offer an option to enter the device passcode after a few Face ID failures, and some even offer it right after the first failure alongside the retry option.
The only places I’ve seen the “Enter Password” option are banking apps which have their own underlying PINs, so they fall back to that instead of the device passcode. Since Avelon doesn’t have its own password or PIN, this expectedly leads to a loop of Face ID attempts.
Avelon version: 1.0.7.3
iOS version: 1.7.3
Device: iPhone 13 Pro
I wouldn’t call it a feature since this is the standard behavior used throughout the system. This includes the example you provided, where Photos will offer you to input the passcode after a few Face ID failures for the hidden album (just tested this). Even arguably much more sensitive functions like Apple Pay payments fall back to the passcode. Those family members can already access all those hidden areas anyway, with the exception of apps that have their own PINs/passwords behind Face ID