So, all my cannons somehow lost their target reticles.
As you can can see with this Sadboye12's Jagenrau II which uses the cannon part as a comfortable rangefinder, the reticle of a cannon is absent. I remember that the "cannon" part had it's own collimator-styled round reticle, but now it doesn't.
Here's older screenshot with my Kamov Ka-56, the green round reticle is visible here.
I always have my predictor setting as "High".
I may have found the cause for this. It seems the round reticle appears only when you're locked on to a target.
Oh wow, just realized mine's gone, too! It was there like a couple of weeks ago before I updated my game (I play on iOS/I may have missed a few updates), so I highly suspect it's a bug brought in with one of the recent updates.
Ground mode shows the line (I'm not a fan of that), but air mode shows nothing.
I've tested
-all predictor settings, with air to air and air to ground
-with function switched from AtA, AtG, and Multirole
but nothing gave me the reticle for Air to Air mode. Quite annoying.