[1.2.2900] Mortars can shoot into mountains

[Lightzy]

Which makes 0 fucking sense and made me rage quit the game.

[Ark]

Please read how to report bugs: https://ludeon.com/forums/index.php?topic=513.0
Mortars vs. mountains is an old topic, and has been studied several times.
https://ludeon.com/forums/index.php?topic=46131
https://ludeon.com/forums/index.php?topic=51084
It is almost certainly a section of thin roof inside the mountain.
A save is required, randomly bombarding mountain bases again is a waste of time.
If you can provide an unmodded save that has a flying mortar shell that is going to punch through thick roof, amazing!
If not, well, you're not the first to miss a section of thin roof.

[Pheanox]

As Ark mentioned, this probably is the game working as intended, some parts of the mountains are not thick roofed, the thick roofed parts are immune.  Without a save, I am going to assume that was the case.