thanks very much for the precious additional edit of your previous message that I just noted now.
This morning I removed the external power supply at 5 V and the transistor driver and fix back all in original condition.
Then I traced the circuit and make the draft in this link: https://we.tl/t-weFrPIjrum
Pls check if it correct.
Then I connected my PQA (logger) for monitoring the + 5V (on C705) that supplies the coil and the voltage on the collector of the TR301 together with the charging current.
Unfortunately the main battery was almost full (and there are some weak cells too) and then the charge current was reducing, a not good testing process! In fact the + 5V remanis stable and the voltage on the collector of TR301 too (close to 0 V apart the starting time). So I could not see any voltage fluctuation.
Now I have to discharge the main battery (using heating system it will take some 2 hrs I have noted, but I want to be there in case something goes wrong...).
On the other hand I fully agree on your sentence " Since you swapped the board and are getting the same behavior, then it is likely some issue with the 12V supply circuit external to the OBC (EV-ECU, fuse, OBC relay, loose terminal in some connector?...)".
But first I have to discharge the main battery, charge at full power (14A), hoping the voltage anomaly will appear again and ask the help of a friend of mine.
However I cannot do this today I hope in the next days.
Surely I will keep informed , hoping this will be of help for someone else.
PS (Edit) I am wondering if the DC supply voltage circuits in the main board (IC317, IC702 etc and relative components) are involved on making the + 5V ? Or such part of DC supply voltage circuits are supplied by that +5V? Unfortunately such components are on the lower part of the main board that is not accessible without disassambling all the main board (action that I have never done till now and I am afraid of doing it).