Thanks for your feedback!
There was indeed something in this brush's controllers that was not working with the flips.
I updated the original file
there and it should now work as expected
This was a very interesting thing to debug because the brush was created before we added the flip function and the controllers in it didn't take that into account.
The full explanation would be a bit complicated and will deserve a full article one day because it is interesting.
The short version is that the "Viewpos" block used as a position reference to snap the strokes at certain places doesn't give the information we need to apply an offset when the canvas is flipped. It had to be replaced by another reference that starts from the real position on the picture (and not on the screen like before) and then apply the change in the view.
We got to make a brush better!