Hi Denis, Jean, sorry for very late response.

Each of you reported[1][2] about unsatisfactory results of
`LaTeX-insert-environmnt' (i.e., C-c C-e) before.

After taking a bit close look at the code, I came to think that the
management of the positions of the point and the mark in that function
is insufficient.  I expect the attached patch fixes the both problems.

Denis and Jean, could you please check it works on your side well? (To
Jean: revert my premature patch[3] befor trying it out)

Best regards,
Ikumi Keita

[1] https://lists.gnu.org/archive/html/auctex/2019-11/msg00009.html
[2] https://debbugs.gnu.org/cgi/bugreport.cgi?bug=35284
[3] https://debbugs.gnu.org/cgi/bugreport.cgi?bug=35284#14