Switching from brush to any other tool and then back resets the brush hardness to 5%.
(edit: this number seems to vary. I think it’s picking the last % number changed? changing spacing to 0 makes hardness reset to 0)
This has been genuinely disruptive to my workflow because it applies to any other tool, including the pan tool and zoom tools, meaning any action other than using my brush causes this event to occur.
I don’t understand why this happens, but the fact that it applies even with the pan tool is an issue that may/will cause it to be a problem for many other artists as needing to switch back the hardness after erasing or panning comes up constantly.