Agree that GitHub shows no significant differences. I'm not a KDK expert and don't fully understand the role of the KDK in the post-install patches, so this is a naive question: could it be that a KDK update might be the reason?
Experimented a bit with 14.2b3 and 1.3.0n (from source, downloaded yesterday, behaviour should be same as 1.2.1).
MBP5,2: the easy case. Installed 14.2b3 over 14.1.1 from USB installer (InstallAssistant available via gibmacos). 1.3.0n in EFI and for root patches. The KDK made for b3 is loaded during root patching. No problems (as always for this machine, needed external keyboard/mouse before root patches are applied).
MBP11,1: as reported earlier in this thread, login loop due to windowserver crash, also here.
(1) installed 14.2b3 to external SSD from USB installer over 14.1.1. Using 1.3.0n in EFI. windowserver crash during login and even earlier during boot. Boot in safe mode ok. In /Library/Extensions there were two extra kexts, AppleIntelHD5000Graphics and AppleIntelFramebufferAzul, left over from 14.1.1
which don't go away when installing over. Removed them manually.
(2) normal boot and login afterwards. No wifi, no graphics accel of course.
(3) applied 1.3.0n post-install patches. No KDK is loaded ("KDK-less" patching for this machine). Root patching re-installs the two mentioned kexts to /L/E. Results in login loop. - NB a CCC data clone also puts them there.
(4) removed only AppleIntelFramebufferAzul and left AppleIntelHD5000Graphics in /L/E. Login ok, no graphics accel. Graphics resolution too high like when both kexts removed or running in safe mode.
(5) the other way around with only AppleIntelFramebufferAzul in /L/E. Login ok, no graphics accel. Graphics resolution low now so there is a visible effect.
So anyway it is easy to recover and avoid the windowserver crashes. Is one of these two kexts the culprit or their interplay with CoreDisplay, SkyLight? Don't know, needs more fine-grained investigation.
More importantly, 14.1.1 on internal SSD runs just fine on the MBP11,1. Big thank you to developers.