With 6.3.0, in the icons only task manager, the tooltips for some items stop working after some (indeterminate amount of) time. Some other items still show tooltips - it doesn't seem to depend on whether the task is running or not (pinned launchers), or whether the window is minimized or not. Restarting plasmashell temporarily fixes the issue, but it reappears after a while. I have been using 6.3 since the first beta and only noticed this in final 6.3.0, it seems to be a regression from beta 2. I see a few task manager commits between beta 2 and final 6.3.0, but can't easily bisect this since it's not clear how long it will take to trigger.
I have a way to trigger this very reliably on 6.3.0 on my laptop. Create a default panel, horizontal. Set the task manager to manually sorting tasks, on click for grouped tasks to show textual list. Open two or more windows that get grouped. Click it to open the textual list, then switch the order of the tasks with drag&drop. After that, when you close the testual list popup, the tooltips are gone and don't come back until I restart Plasma. With this, I have basically 100% reproduction rate in stock Fedora 41 6.3.0 and on self-compiled 6.3.0. It no longer happens with 0ed639e4f99ecfb4e3f46512f205f123ba71b244 or with master (and I haven't seen it happen on master in daily use, while it did happen on stock 6.3.0, and I think I saw it happen on master at some point in the last couple of weeks but can't exactly place when). I think that's a good indication this is fixed in 6.3.1 with the above commit. Please reopen if it still happens in 6.3.1 once it's released.
Ah yes, I can consistently reproduce it just by hovering over the task list popup. Will check with 6.3.1 today.