That is a very good point, lckent. As it happens, my wife and I have drained that account (they never turned off my card, just my wife's) so we got the money anyway. In our circumstances, moving custodian would, I imagine, have meant that the balance minus the disputed expense would have been transferred. The fact that they didn't turn my card off is surprising, as I sit here typing this, but I'm not going to complain.
I have had arguments with HSA and FSA custodians regularly over the years, on a variety of topics and, until the idea of "authorized" establishments came on the scene, it was exhausting to argue with them, especially if it was an FSA in the grace period. My wife has just changed jobs so we will find out, in due course, what the new lot are like to deal with.