Reproduction:
1) Go to "Options" -> "Other" and DISABLE "Automatically enable new mods".
2) Go to "Mods" -> "Install Mods" and install any new mod.
3) Press "back" after download finishes.
Result:
The game restarts, but the new mods are (correctly) not enabled. Thus the restart didn't do anything useful.
Expected Result:
Pressing "back" should just go back to the previous menu without restarting the game.
Addendum:
On a personal note it probably wouldn't hurt to relabel the "back" button to something like "back (restartI)" whenever applicable. The sudden restart can otherwise produce a quite startling screen flash
.