I think the error you ran into is the same heisenbug we’ve been trying to chase down for over a week: two PCZTs that should be mergeable fail to merge. We don’t yet have a reproduction of it on a developer phone, and we can’t have users’ wallets send us that reproduction when it occurs because it’s sensitive transaction data. Fortunately as you found out, it only happens randomly, so trying to create the same transaction again works.