Hi,
Reading the documentation for the variable you just pointed to made me realize that maybe transient-mode is somehow being turned off.
I did `emacs -Q`, and checked the value of `transient-mark-mode' in the customize interface. It says that the value was "modified outside customize".
So, something is setting this variable to nil, even though the standard value is `t'.
No idea how this could be caused (maybe it's caused by the emacs-plus homebrew formula I'm using?) but at least I can get rid of the weird behavior by turning on transient mark mode now.
Thanks a lot everyone for all the help. I really appreciate it.
Best,
Parham