Hi,
sorry if it's a known problem.

i ve noticed that if you change the brush size, then use the opacity slider on the toolbar (with the mouse, haven't test it on my tablet yet), the brush size is resetting to the default size... i've tested it on the 4.2.1 and 4.2.2

I think i've found why : the problem seems to be on the size slider, i use "shift drag" to increase the size of the brush but the size slider is not updated so when i change the opacity or flow, it takes the size slider value which haven't been updated, and update the size brush.

This bug seems to be related: https://bugs.kde.org/show_bug.cgi?id=408331

You might want to add a comment with your information.

Thanks @ahabgreybeard i've made a comment.