I also have a similar problem to yours. From a fresh install from ISO the top latte panel's translucency is erratic. This happens on 1 physical machine and 1 VM, but very very very rarely happens on my primary machine (thank god).
I believe this has something to do with inconsistencies in latte's features. Now how do I paste a picture again so I don't get fired at by doing it the wrong way.... ha, funny, I didn't find anything searching the forums nor the wiki, I must be looking in the wrong place. Oh well let's use some elbow grease then:
Right-click on Latte's top bar, APPEARENCE tab and then at the bottom Prefer opaque background when touching any window + Hide background when not needed + Busy Opacity slider a little bit up.
These settings will play a role on how the panel's translucency behaves.
See if any change there suits your needs. If it still doesn't behave as it should, I suggest to open an issue at latte's github page. Oh no wait, they moved it to KDE bugtracker now Log in to KDE Bugtracking System