Is there any logic to what the next mag selected is based on where it is stored? Or perhaps the order it was picked up etc?

For example, having the fia ak with the two mag types fmj+tracer only. Putting the fmj mag in different slots (shirt, pants, chest rig) it seemed to not matter and would always choose tracer mag upon pressing R.

I assume there must be some logic to why this is happening though. If I ever find it I will update this post.

I think I got it.
I took 3 of each type type magazines for SVD. 9 magazines (didnt take FMJ coz it has not own icon on HUD). In vest they are located they are located in the following order
3x(10/10)[sniper] 3x(10/10)[tracer] 3x(10/10)[API]
Then I shooted round and made this combination
(10/10)[sniper] (10/10)[tracer] (10/10)[API]
(7/10)[sniper] (7/10)[tracer] (10/10)[API]
(4/10)[sniper] (4/10)[tracer] (10/10)[API]
Now if I press R it loads either 10/10 sniper or 10/10 tracer FOREVER because:
-they are full
-they lies before (more left) API magazines in vest grid.
If I shoot 1rnd from 10/10 tracert there is no more 10 tracert. Whats next? Right, its 10API magazine. Now it starts loading either 10sniper either 10API.

So how it is
it prefers
-fuller magazine
-more first (more left in inventory grid) magazine

How it should be
it should prefer
-fuller magazine
-of the same type

You can say that there are situations when you are in the combat, enemies everywhere, you need next magazine and system loads 10/30 rnd magazine of the same type instead of any other full magazine... But I didnt meet those situations. But situatinos when I need to be stealth I start shooting tracers I meet pretty often.

I didnt test between shirt/pants/vest.