SUMMARY
I am using a Plasma theme with 1) an overlay texture on all background elements and 2) rounded corners.

1) After every reboot, Plasmashell fails to apply the overlay. Every other part of the theme (except for rounded corners, see point 2 further down) is rendered as expected but the overlay is missing.

I have a shell script I use when developing my Plasma theme, which stops Plasma, purges its cache and restarts it so I can easily test every iteration:

#!/bin/sh
pkill plasmashell
rm ~/.cache/plasma_theme_*
plasmashell &

Running this script manually after every reboot is necessary to have Plasma correctly apply the overlay from the theme (it is especially obvious that the overlay is missing when opening a tray popout or the calendar). After doing that, it continues to work for the duration of the session.

2) After every restart of Plasmashell, popout windows (e.g. the calendar, tray widgets, etc) that touch a side of the screen do not have rounded corners but square corners. The shell script above does not fix that. It can only be fixed (and only for the current run of Plasmashell, it will come back the next time it restarts) by using Alt+Drag on the popout window to move it away from any side of the screen. Once this has been done with at least 1 such widget, it's fixed for all of them for the rest of the session.

STEPS TO REPRODUCE
1. Use a Plasma theme with an overlay image and rounded corners (for example: https://gitlab.com/obnosim/velours)
1. Reboot and log into Plasma (X11)
2. Open the calendar or a tray widget

OBSERVED RESULT
1) The theme's overlay image is not applied until Plasmashell is restarted manually.
2) The theme's background corner elements are not applied until the window is moved away from the edge of the screen.

EXPECTED RESULT
1) The theme's overlay image should always be applied.
2) The theme's background corner elements should always be applied.

SOFTWARE/OS VERSIONS
openSUSE Tumbleweed, x64, X11
KDE Plasma Version: 5.21.5
KDE Frameworks Version: 5.82
Qt Version: 5.15.2

ADDITIONAL INFORMATION
I'm not sure when this started happening, it could be back since last fall.
I don't know if this is also happening under Wayland.