The reason for the split was that I stupidly took the panel from my
workshop, where the humidity had been 70 to 80% for the prior 3 to 4 weeks,
into my centrally heated house (RH 25%) in order to finish the assembly.
(need a bigger workshop !) Worse, I left it near the radiator over night -
next morning the split had appeared. I moved it back into the workshop and
the split completely closed up. I am now slowly conditioning the the panel
to my house conditions, so far, so good.