So, I'm not customer service here or anything, but this problem sounds weirdly familiar to me. The underpinning tech of FFXIV's Mac port is the WINE-based Crossover system from CodeWeavers, and there's an issue I've seen before with things running in WINE in full-screen mode: if you have the Windows app within a WINE bottle running in 'full-screen' mode (even if that 'full screen' is actually treated as a window in the host system), it basically has to treat the window for that app differently and there's a bug where flipping out of that window and back can cause it to lose the ability to register mouse clicks. It would not surprise me in the least if the WINE issue in question is exacerbated by the hoops through which Crossover now jumps to work on Catalina.

Short form: if it were that bug, switching to another app would have a chance of making the FFXIV window no longer register any mouse click events. (And possibly would stop it from even showing tooltips if you hovered over things.) I don't have the macOS version handy to check, but if the Mac version has an option in the display menu to run in windowed or borderless windowed modes and is currently running 'full screen', I'd try switching modes just to see whether or not it improves things at all.