ICon-only task manager icons become smaller on startup. entering and exiting edit mode fixes it
Same issue here. Plasma 6.0.4 on Manjaro. Turns out trying to take a screen shot is another method to "fix" it
Every time I start my kde session, or restart plasmashell, this bug appears. But now they are not smaller, they doesn't show at all, only separators. Printscreen or edit mode fixes it.
*** This bug has been marked as a duplicate of bug 489086 ***