I think I found the problem. If you go to the 17 pounder gun EBPs, UI_ext go to selection_type and set it to squad_select from no_select. I figured it out, eventually, because my recent mod has the 17 pounder pit for the 25 pounder arty piece which had the aiming cone. So from there, I knew it was the gun and not the pit. Then it was left to comparing the two.
So, I tried the solution and it mostly works. Except for one thing:
Apparently, if I set squad_select to the 17 pounder gun, it no longer becomes possible to select the squad garrisoned inside it