I had this bug also but it appeared to go away on the next level. I kept picking up ammo just in case, it eventually got out of the red and upon next level load, was normal
I've had the limited scar ammo since I first picked up a "Ranged scar" on Lab rat & its been like that through Gate keepers & dead man walking and its still like that on seat of power which I have just started.
It really didnt matter to much as I was having great fun abusing the Cell with the stealth nova but now that I' fighting the Ceph its brutal not being able to carry more ammo.
Also noticed when doing "Sudden impact" on this play through when I was using the scarab it it wouldn't give me ammo of the scarabs on the ground.
What I had to do was switch my scarab for the one on the ground then change its attachments to the same as the one I just dropped to get ammo from it....But!
After a while whenever I did this instead of giving me ammo it started to remove 10 bullets from my reserve till I was a t the point that I could only have 70 scarab bullets in reserve.