Did a bit or playing with modern Magisk, and it seems we can no longer 'downgrade' the App to a version earlier than running Magisk daemon in 25100+...
Force app version not lower than daemon
vvb2060 authored and
topjohnwu committed 22 days ago
So with Magisk 25101 loaded, there is
no way I can downgrade the App to Stable 25100 (despite the binary being in sync), or run any earlier app (Including old Manager app) on my device.
Whether calling apk package from App by setting Channel to Stable (App not hidden), or by downloading this or earlier Apps (previous App uninstalled), all the packages will simply install the stub app which in turn prompts:
and this always fails with never ending:
until it is force-closed.
(I guess
Build dynamic stub resource APK at runtime …
topjohnwu and
vvb2060 committed 17 days ago
or
Fix app state,
vvb2060 authored and
topjohnwu committed 7 days ago
may fix some broken behaviour.)
At present, the only Magisk App I can seem to install successfully (w/ 25101 binaries loaded) is 25101!* ... This installs the full App (not stub) as expected.
... It seems we must at least remove root injection and do an initial re-flash again just to install synced Beta/Stable App, and that any other 'downgrade' is also precluded...
*Further, I've found I can no longer even install a CI App, even one later than my Canary build (these actually use the same last app version, ie. also 25101), from Actions (Continuous Integration test build) runs with 25101 Magisk binary installed!

... Same behaviour as above... I'm guessing this is due to app signature checks.
If anyone wants to try, or is able to downgrade the Magisk App from current Canary or Debug to Stable, or to upgrade this to a CI build w/o restoring unpatched images (removing root), please report... I'll reward you with some happy faces...

PW