While reading a Maildir folder with neomutt I accidentally saved mails
into an existing mbox file rather than another Maildir as I meant to,
but when I opened that mbox file I could not see these mails (and not
with mutt either).

Debug log shows that the date in the header is wrong:

[2020-03-28 20:16:18]<1> is_from()  expected weekday, got: Sa Mär 28 02:33:48 2020

[2020-03-28 20:16:18]<1> is_from()  expected weekday, got: Mi Feb 12 11:45:21 2020

[2020-03-28 20:16:18]<1> is_from()  expected weekday, got: Mi Feb 12 05:07:36 2020

[2020-03-28 20:16:18]<1> is_from()  expected weekday, got: Do Feb 13 13:45:56 2020

As you can see, the dates are localized to German which they aren't
supposed to be. This does not happen when I save with mutt instead, or
when I use neomutt to save from mbox to mbox.

I'm not sure about the timing, but the same mbox contains good messages
from when I saved to it earlier, so possibly this bug appeared only in
the latest neomutt upload.

Marked this as important since it constitutes a sort of silent data
loss. The messages aren't really lost (they can be restored by manual
editing of the mbox), but neither mutt nor neomutt even make any mention
that they are not displaying messages because of errors outside the
debug log.

Did you try it with 20201127? Is it solved?

I did (with 20201127):
 * Open a Maildir
 * Pressed S
 * /tmp/mbox (mbox file for testing)
 * Open /tmp/mbox

When I open the mbox-file, I can see my mails stored in the mbox-file.
(But I see an addition From line in, wehen I open the message)