gvim 8.2.2267 (Big) with GTK3 GUI; 'encoding' = utf-8; 'filencodings'
(plural) = ucs-bom,utf-8,latin1

Steps to reproduce:
1. Load a Latin1 file containing only characters in the range
0x00-0x7F. (In my case this was a corrupt version of the file.)
-- 'fileencoding' is set to utf-8, which at this point is acceptable,
since the de-facto encoding is us-ascii, which is byte-compatible with
both Latin1 and UTF-8.
2. Replace the file-on-disk (by non-Vim methods) by a version
containing one or more (Latin1) characters in the range 0x80-0xFF. (In
my case this was the correct version of the file, after fetching it
over the Net.)
-- Vim gives a prompt, with options [O]K, [L]oad file
3. Answer l (Load).
-- File is reloaded, but the 'fileencodings' heuristic is not
reapplied: 'fileencoding' (singular) is still utf-8, any Latin1
characters above 0x7F (which are not valid UTF-8 byte sequences) are
changed to question marks. No error for invalid byte sequences (I
didn't notice any at the time, and none is recorded in the :mess
messages list).
4. Make some more changes inside Vim, adding more characters in the
range 0x7F-0xFF, then save.
-- File is saved as UTF-8; if read as Latin1 outside of Vim, weird
characters appear where changes were made at step 4. <-- bad
5. :setl fenc=latin1 | w
-- If reloaded outside of Vim, the weird characters have now
disappeared; but the question marks, if not replaced by what they
should be, are still there.

Best regards,
Tony.

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