The GFCI's here are quite sensitive, the max current unbalance allowed is 30 ma. As you say , it may trip on far less, and they do. The wires in metal tubing has an capacitive unbalance. The transformer is pretty strange seen from the experience of modern systems. It is so called IT system, and that could be looked at as a US 240V delta system where you forgot the grounded Neutral! All voltages are floating in hence of ground!
The fault was made by a frequency converter, The converter was well grounded, but the local GFCI did not release, but it is tested and OK. A new converter solved the problem, so my guess is that it was a non mains frequency fault.
That is so far I can get in this case.
https://www.electricalclassroom.com/ungrounded-system%ef%bf%bc/
