Opus just calls ReadFile until the file runs out.
It sounds like Serena Network drives return the wrong data, adding extra zeros on the end which are not meant to be there to pad them out, maybe to a multiple of a sector size or something similar.
If so then the problem is with Serena. Those are incorrect filesystem semantics on Windows.
Given previous issues with Serena Network drives not implementing basic filesystem semantics, this seems a likely explanation for this new problem.
I would report these issues to the Serena Network team as their product appears to have some incorrect behaviour.