CO2 levels rising...and rising...and rising

[That's not my dog]

So, I am on patrol and I am diving and surfacing with the rising and setting of the sun in the China Sea. Planes everywhere! Anyway, my CO2 level kept rising and was not going down when I surfaced. I would be cruising at 10kts and my batteries would recharge, but my CO2 still kept rising as I was surfaced. Interesting thing as well was no one was going outside to stand watch. I put a guy on the AA gun and he never showed his face.
I kept cruising along and eventually the whole crew died of CO2 poisoning while I was surfaced.
Now for another twist, I reloaded that game and instead of my CO2 being at 45 when I reloaded it the first time, it was zero! I think it has something to do with saving my game while submerged. not 100% sure, but a hunch.
Anyway, just seeing if that has happened to anyone else.
Happy sailing.