This is very minor, and may be a Thunderbird issue, not related to Plasma. After a recent round of updates (Kubuntu 24.04 Plasma 5.27.12, x11), after I launch Thunderbird (a snap on my system), its task manager icon is blank, or blank with a small ‘+’ at the bottom when grouped (i.e., when there are multiple Thunderbird windows open). The launch icon (in menus, and pinned) is normal. But not the task icon. Other apps don’t have this minor symptom, so maybe it’s T-bird.
With every iconpack?
I refreshed my snap to v143.0.1-2 (monthly/stable but not esr). With this version, launching Thunderbird no longer produces a second icon in the task manager, as it did for a long time. (Until recently, the second icon was normal. A recent update made it blank.) Now, the pinned icon looks fine and behaves correctly after launch. The task manager Thunderbird icon also appears correctly when the application is not pinned. Seems the problem was ephemeral, and tied to a particular Thunderbird snap.