Thanks for all the comments about this. Given what happened and what I've learned since I wouldn't recommend hookless for a tandem either. Having said that I'm not sure I'm going to get rid of them - yet. I ran tubeless on 650b hooked rims with 38 mm tires on our previous tandem at 60-65 psi - and there is more volume in the 700c x 38 tires so running 55 psi seems fine.
BTW to JulesCW the tires were definitely seated when I pumped them up - they were just a touch soft and I usually check pressure and top them off before every ride.
FWIW the Paris Roubaix race last weekend was won on tubeless tires which almost every team ran this year. Dylan van Baarle was on hooked rims, but nearly half of the teams were on hookless. Not to say that hookless on a tandem is the right setup, but to point out that there is certainly no consensus at the very highest end of the sport about hooked vs hookless.