Went from framework 5.9~ to 5.10~ and now dashboard only displays on a black, opaque background. All desktop effects settings have no impact. Installed a transparent theme and still no effect. I have kde plasma installed on 4 other machines (3 on Antergos and one on Manjaro) and none have this problem. The problem machine is an old core 2 duo running at 2.0 ghz with 4 gb ram.
Not sure, can you talk to your distro about it? It might be drivers, Mesa, ... there were no code changes on the Dashboard side.