Will having different macOS affect Mail?

[ace (Adam Engst)]

@Simon

My understanding so far has been that the bug is related to On My Mac mailboxes.

Alas, no. See:

Apple Releases macOS 10.15.4 Catalina, watchOS 6.2, tvOS 13.4, and iOS 13.4 for HomePod

The remaining Mail data loss bugs are not limited to local “On My Mac” mailboxes. The most common issue I’m hearing about with macOS 10.15.4 is dragging and dropping a message to a mailbox on an iCloud server. More info here and here.

Also…

@neil1

From anecdotal net research…the problem involves gmail which I don’t use.

There may be issues with Gmail as the server, but at least some of the problems (as noted above in @mjtsai’s post) involve mail hosted on iCloud.