When I call describe-key and enter … (Alt Gr + .), it says M-b runs the command backward-word […] and async-shell-command (bound to M-&) is called.
I'm on Linux with Xorg, the terminal emulator is Alacritty, keyboard layout is German. I can enter … just fine in nano.
showkey -a shows this when I enter …:
$ LC_ALL=C showkey -a
Press any keys - Ctrl-D will terminate this program
… 226 0342 0xe2
128 0200 0x80
166 0246 0xa6
^D 4 0004 0x04
Edit: Additional information ↓
This is what view-lossage reports:
M-b ;; backward-word
C-M-@ ;; mark-sexp
M-& ;; async-shell-command
Interestingly, this happens only if I open the file with emacsclient without options. When I open the same file with emacsclient --tty, … is correctly inserted.