File resource checksum => mtime fails with empty mtime

In order to improve performance for large binary files I have been using File resources with the checksum type set to 'mtime' for a while and I was satisfied with it . But apparently, this does not always work. On a different system, I keep getting :

Error: File written to disk did not match checksum; discarding changes ({mtime} vs {mtime}Thu Sep 13 16:19:24 -0400 2018)

As if the mtime on the server could not be compute.

This seemed like a bug.