If combinerawtransaction is given a transaction containing completed taproot spends, it will drop the signatures from those inputs. For 0.21, combinerawtransaction would actually cause an assertion failure - this was fixed by #21330. This is not a regression as combinerawtransaction never worked with txs with taproot inputs anyways. AFAICT, either taproot was not yet implemented, it hit an assertion, or it is broken.
In any case, I don’t think this is particularly important to fix since the recommended workflow is to use PSBTs. But I am opening this issue to document this in case someone does come across this bug.
Originally found by @roconnor-blockstream
