Rotating an encryption key involves re-encrypting and re-uploading every item you have stored (vault items, folders, some send data) and then storing the new encryption key. If this full process does not complete, you’re in hot water.
The most likely ways for an issue mid-rotation would be either network issues or closing the client in the middle of the operation. In principle a Bitwarden server crash would do it, as would a client crash, but network or forced application stoppage are more likely.