5K iMac as secondary display no longer allowing 5K resolution after Catalina

I recently upgraded my primary Mac to Catalina (my secondary mac was already on Catalina). Whether causal or coincidence, after my upgrade, I noticed my 5K iMac was no longer allowing me to use the highest available resolution.

Screenshot of secondary machine host OS showing that it is capable of 5K

Screenshot of secondary machine with Luna Display Secondary running showing that 5K is not offered

That means: when you in 10.14 macOS version, luna support 5k display?
Sorry about this question, because I didn’t test in 10.14 version, I’m in 10.15 catalina and have same problem with you.

I am also using an iMac 27" 5K (2017) on Mojave as secondary display and, although the resolution is the native one at 5K, is not shown as Retina. AFAIK 4K/5K Retina is not yet supported by Luna Display on any macOS version. Check this thread.

@harris you could try using external software to change the native iMac resolution to 5K, and then launch Luna secondary app, should work (although no Retina support).