I opened an encrypted GnuPG file (.gpg extension) in an xterm via SSH
(no X11 forwarding). After typing my passphrase in the terminal, the
file was opened successfully, but the character "e" was inserted at
the beginning of the buffer, and the PageUp/PageDown keys were not
recognized.

I suspect some bug in the terminal initialization code for xterm.

This occurred in the past with slow connections, but at least with
normal files:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1016043

(perhaps this in unrelated to GnuPG and this is exactly the same bug).