I’ve seen that behavior, @richl, when I replace a sheet in the middle of a dependency chain (including when performing a Restore in the Tiller Community Solutions add-on) like replacing Accounts in this chain:
Balance History → Accounts → Balances
(This is a flaw in the add-on functionality. I’d fix it if a) I had a solution, and b) the problem happened more frequently (but most of our intermediate dependencies are pretty stable sheets).)
I think that happens because our Restore process deletes Accounts— essentially breaking the references back from Balances— then inserts a new sheet with the same name… and without the formula jostling Sheets doesn’t quite realize the INDIRECT() reference back from Balances is functional again.
I haven’t seen these formulas fail when inserted as a new sheet. I find them pretty reliable.