I'm using two keyboard layouts: English and Czech. I've set "Switching Policy" to "Application" and I've also assigned a custom global shortcut (Meta+K) to layout switching (I don't know whether it's relevant). What happens from time to time (but definitely at least ten times a day, probably more often) is that when I switch to an application that had English layout the last time it had focus, now it suddenly has Czech layout. I actually can't remember for sure whether this happens when switching between applications on the same desktop but it definitely happens when switching between applications by switching desktops (again: maybe this is relevant, maybe not). Unfortunately I haven't found an exact way to simulate this, it just "happens". Reproducible: Sometimes Steps to Reproduce: See above. I don't know a reliable way to reproduce this.
I haven't noticed this behavior for at least a few days (maybe even weeks), so it seems fixed in 5.5.4. I've always suspected that it might have been somehow connected to https://bugs.kde.org/show_bug.cgi?id=356220 which is also already fixed (but maybe it's a coincidence).