The icons-only task manager doesn't show desktop actions for some applications. It's a hit or miss situation. One case I can site is that Firefox still shows the desktop actions both in application launcher and the task manager, while Pale Moon only shows the desktop actions in the application menu. Another example where actions only appear in the launcher is Thunderbird.

I was gonna file a bug, but I don't know which product I should choose.

I'm currently using Plasma 5.16 on Kubuntu 19.04