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Unable to 'move window' to another desktop

I’ve a few small issues with an updated recent install of 19.04 Rolling and I’ll put them in separate posts because they’re unrelated. As an aside, it might be a useful feature if in the ‘About System’ window there was a ‘copy information to clipboard’ button; that would make it easier to just stick that information into posts.

Older Radeon graphics card, OpenGL driver R600G.
Dual monitors, different resolutions: 1 @ 16x9 1080p, 1 @ 4x3, desktop expanded across both monitors.
With Spotify maximised in the 4x3 monitor on desktop #4, I try to move it to desktop #1 but it stays where it is regardless. I tried with the context menu on the title bar (move to desktop 1) and while on desktop #1 with the context menu from the taskbar (move to current desktop). Both methods have no effect and the Spotify window stays where it is. I would expect it to move to desktop #1 and be in the same place there, maximised on the 4x3 monitor.
If there’s any information I can supply let me know.

I may have a partial answer to this problem.
The Spotify window has what appears to be the X-Windows stylized X in the top left corner. So I’m guessing that it’s not running in a standard KDE window, or something like that. I don’t know the details behind all of the windowing magic, but every other window seems to behave and they all have their custom icon in the top-left corner.
Maybe this is incorrect, but it’s the only theory that I’ve been able to come up with.