I have experienced this issue. You can see in my (still to be re-updated) signature my setup. Of note, though, is I had to do a clean install after my backplane failed and I had moved my primary hard drive to my Mac Mini. So within the last week, I got this machine started from scratch. And all seems to be well, except for the sleep issue and kernel panics. Today I turned sleep to "never" and the machine has hummed along just fine.
I'm on 10.14.6 with all available updates loaded. I do not have the Northbridge flag in the kernel panic.
Which Mojave Build No. do you have installed ?
You probably are aware of how to find the build No. but just in case . . .
There were so many builds released shortly after the first 10.14.6 release.
Before the last Security update my Build No. was 18G103.
I did a clean install of Build 18G103 to a spare APFS formatted SSD with NO 3rd party apps,
I cloned this drive to ANOTHER SSD formatted to HFS+.
I then shut down, removed the power cord for 15 mins.
Re-seated all my RAM & GPU did the THREE CHIME NVRAM reset.
Re-booted to the HFS+ SSD and let the cMP sleep for an hour .. . several times.
Woke from sleep without issue every time.
I have since cloned this install to a HFS+ Samsung 970 EVO.
I have had zero sleep or other issues since.
NOTE : The last Mojave Security Update changed my Build No. to 18G1012.
No idea if all of this negates the sleep - kernel panics but my cMP seems to have escaped these problems and I do have put "drives to sleep whenever possible" set.
Hope this is informative.