npostavs@users.sourceforge.net writes:

I have tracked the issue down to scan_sexps_forward in syntax.c

Applying this change which reverts part of [1] seems to fix the issue:

Alan, can you explain what the idea behind that change was?

I think it might correspond to this part of the commit message:

    Reformulate code at the top of the main loop correctly to
    recognize comment openers when starting in the middle of one.

[1]: 9dcf5998935c Sun Mar 20 13:19:48 2016 +0000
"Amend parse-partial-sexp correctly to handle two character comment delimiters"