Just FYI…
I was having some external monitor issues lately, so I rolled the dice and upgraded from 22.04 to 24.04 beta on Thursday. This is my all day work+personal device and it has been working flawlessly so far with suspend/resume and external monitors. No issues yet.
You using a dock?
Didn’t have issues with external monitor. Only when powering on and plugged to the dock, the monitor is not discovered. Hav to unplug the USB-C and re-plug for the screen to appear.
There are 4 patches in v6.9 that might help this issue. You can try to cherry pick them and see. They’re not stable candidates though, I don’t see them being backported into kernel 6.8.
Thx for the hint. I’ll however probably wait until there is a 6.10 HWE available for KDE Neon/Ubuntu. My times where I did build my kernels is long past… I just don’t have time anymore to spend lots of time on these things anymore. 4 Kids take a heavy toll on your available time 
They’re merged in 6.9, so if you want to try the Ubuntu mainline PPA you might be able to see if they work for your issue without compiling a kernel.
But yeah otherwise waiting for them to land in Ubuntu kernel is just fine too.
For sure, the monitor issue (through a dock) persists in 24.04 beta. I have been using it for a week all day and it is certainly stable enough to use, but the external monitor issue through a dock is not resolved. Thanks to @Mario_Limonciello for pointing out some 6.9 kernel updates that might address it (I havent tried yet).