Hello. I'm using pen tilt (Tilt direction) for brush rotation and got some strange behavior since 4.2.5: canvas rotation now also affects brush tip (with rotation setting checked), making it unusable. Here's visual explanation: https://i.imgur.com/DLO0Kzq.png
Is it a bug or am I doing something wrong?

I don't have a pen tilt sensing tablet but I see this effect happening all the way back to version 4.2.1. It doesn't happen until it's been 'induced' by using the Rotation (canvas rotation) control input in the Rotation parameter, using the brush preset and then deactivating that control input (by replacing it with a different control input). Also, as in your linked image, the brush tip rotation is about 2x canvas rotation. What you're seeing may be the same or a related thing.

Did you activate the Rotation (canvas rotation) input at any stage for the Rotation parameter on that brush preset?
Does the effect go away if you Reload the brush preset then reapply your Tilt Direction control input?
Does it go away if you Reload the preset, restart krita then reapply your Tilt Direction control input?
Are you certain that this only happens with 4.2.5 and does not happen with 4.2.4 and previous versions? (You can easily check this by downloading and using the portable .zip package of a krita version from the stable library: https://download.kde.org/stable/krita/ )

Yep, just yesterday I found out that in 4.2.3 it's also buggy (fortunately, it only happens when you change settings "on the fly", so it doesn't matter). But in 4.2.5 tip rotation is persistent - it doesn't jump back when you rotate the canvas.
I also have to add that pen tilt works fine and brush rotates (2x canvas rotation) as soon as you check "Rotation" setting even with mouse.
I can't test more atm but hope that's enough info to figure out what's going on.

The situation looks to be even more complicated than we've discussed here and may take quite a bit of sorting out. This is best investigated by the developer team of course.

Can you raise a bug report here: https://bugs.kde.org/ (registration and log in are needed)?
Just give a straightforward top level description of the symptoms and how they are brought about. Please fill in information according to the drop down lists and the sections given in the bug report template.

I'll try to file it when i'm back on desktop in a couple of days (not quite comfortable to do it on a phone). Thanks for your response.

There is a new bug report which I think is the same issue: https://bugs.kde.org/show_bug.cgi?id=410583
If that's what you're experiencing, it would be appreciated if you could confirm it too and attach that screenshot.

Okay, that bug was fixed. It should appear in the next bugfix release and the nightly build starting tomorrow if you're impatient.