Well that would be frustrating. I haven’t experienced that. But two things that may ne somewhat related.
If I preview a mail and reply, then if the original message did not finish downloading content then it seems temprarily unavailable in my reply. Which I have had same issue in MS Outlook, fairmail etc.
Occasionally hitting reply takes sometime for email chain text to load. initally it appeara missing but show up as I type.
And obviously if I don’t hit load remote content that content is not available.
It is possible it is device related?
Did you try Beta App that may have fixes?
Hmm, is there an alternate mouse option of save as?