This issue is from a related discussion on Apache httpd, which was trying to comply by sending a 504 even though the revalidation error was a 502. That's nuts.

The current text for must-revalidate says:

The must-revalidate directive is necessary to support reliable operation for certain protocol features. In all circumstances a cache MUST obey the must-revalidate directive; in particular, if a cache is disconnected, the cache MUST generate a 504 (Gateway Timeout) response rather than reuse the stale response.

However, if a proxy/gateway received a 5xx response while trying to revalidate, this requirement forces it to toss that information and supply a useless 504 instead. That isn't desirable. The entire purpose of this requirement is to make sure that the cache does not respond with its stale entry. Hence, any 5xx code is sufficient.