Ok, now it happened again. Exactly the same thing. After a night of hibernation, without being on the charger, the LCD does not come back.
Same as before. Of course can still be a connection issue, but might also be systematic.
I will explore some more next few days and report back.
However, if any reason for such a failure is known or likely I am interested to test and verify.
In the meantime some logs. This is for a successful resume:
[ 1.951191] i2c 2-005c: Driver it6251 requests probe deferral
[ 4.003890] it6251 2-005c: System status: 0x3e
[ 4.011898] it6251 2-005c: RPCLKCnt: 2720
[ 4.019552] it6251 2-005c: Clock: 0x193
[ 4.026230] it6251 2-005c: Ref Link State: 0x00
[ 4.032875] it6251 2-005c: eDP system is already stable
...
[ 1348.456706] PM: restore of devices complete after 8613.293 msecs
[ 1348.804694] it6251 2-005c: System status: 0x3a
[ 1348.819925] dummy 2-005e: it6251.c:it6251_init:277 error -1 writing 255 to 5
[ 1348.932925] it6251 2-005c: System status: 0x3e
[ 1348.939068] it6251 2-005c: RPCLKCnt: 3728
[ 1348.944809] it6251 2-005c: Clock: 0x193
[ 1348.949468] it6251 2-005c: Ref Link State: 0x00
This is for a failing one:
[ 1.951235] i2c 2-005c: Driver it6251 requests probe deferral
[ 4.012289] PM: Using 3 thread(s) for decompression.
[ 4.012289] PM: Loading and decompressing image data (205830 pages)...
[ 4.243955] it6251 2-005c: System status: 0x3e
[ 4.251593] it6251 2-005c: RPCLKCnt: 2208
[ 4.257163] it6251 2-005c: Clock: 0x193
[ 4.258346] PM: Image loading progress: 0%
[ 4.266202] it6251 2-005c: Ref Link State: 0x00
[ 4.272587] it6251 2-005c: eDP system is already stable
...
[ 148.826679] PM: restore of devices complete after 8602.114 msecs
[ 149.174443] it6251 2-005c: System status: 0x3a
[ 149.191095] dummy 2-005e: it6251.c:it6251_init:277 error -1 writing 255 to 5
[ 149.627906] it6251 2-005c: System status: 0x3a
[ 149.632363] it6251 2-005c: Display didn't stabilize. This may be because the LVDS port is still in powersave mode.
[ 149.643327] it6251 2-005c: Will try again in 400 msecs
[ 150.053909] it6251 2-005c: System status: 0x3a
[ 150.066887] dummy 2-005e: it6251.c:it6251_init:277 error -1 writing 255 to 5
[ 150.529913] it6251 2-005c: System status: 0x3a
[ 150.534440] it6251 2-005c: Display didn't stabilize. This may be because the LVDS port is still in powersave mode.
...
So it seems early boot thinks all is fine with the LCD, but then after restoring from disk, the old kernel state thinks it is faulty.
The difference being that a short hibernation is able to reset it after the first fault, while a night hibernation cannot.