Android still allows apps to detect what other apps are installed? That's as much an OS problem as an app problem, assuming you didn't have to consent to some kind of device management profile first.

It's not unreasonable that an app developer would flag a sideloaded version of a password manager. It likely points to problems far more often than it points to people enacting extra-strong security controls or doing legitimate local builds. But it's not reasonable if the OS enables this capability by default.