It could be a fault in the hot tub, given it holds out on a kettle. Can you IR test the tub at 250V (just in case electronically switched) to check that L+N to E is showing tens of Mohm?
Another possibility is a N-E short somewhere (not necessarily on the new circuit). If you are on a TN-C-S supply the origin N-E voltage is practically zero so typically that only shows up if you have loads of tens of amps or more. It might be the kettle at ~10A is not enough to cause enough N-E current to divert, but the tub coming on pump and heaters at 30A (or more, if motor surge is simultaneous, etc) is enough to reach threshold.