I'm not sure I should create a new bug report, it's similar to https://bugs.kde.org/show_bug.cgi?id=322344 The difference is, all tilt input (X, Y, direction and elevation) reverses only after I make some change to the brush, like swapping the brush tip while the canvas is mirrored. It also only seems to affect brush rotation. I tested opacity and size controlled by tilt and those behaved fine. Flipping the mirror direction fixes it. Happens in 4.1.7 as well as current nightly 4.2.0-pre-alpha (git be454f9).
I can confirm this bug Tilt gets back to "normal" during mirrored mode if brush tip settings are changed during mirror mode WORKAROUND: exit mirror mode and reenter.
The bug is not reproducible in master, 4.2.9 Beta1 and 4.2.1. The only way how I managed to reproduce something like that, is pressing 'M' button right in the middle of the stroke. But that is expected behavior. I'm closing the bug as fixed. Please reopen it if you can still reproduce it.