Hello Benjamin, any temperature greater than 65C is going to unfavourably impact moisture content in meat, no matter the cut.
You can ignore the water tank. The steam produced in your oven changes to water vapour and vents out of the oven. Convection adds to loss of moisture. Steam speeds heat transfer, but does not add moisture to meat.
2cm of water in a container is not a helpful detail because you don’t reveal how much meat your are cooking.
1 kg meat can be expected to give up about 90 - 100ml, or more depending on cooking temperature.
Beef cheeks are not an easy item to cook well. Next time try something less challenging. Maybe a chicken leg?
How do 24-Hour Beef Cheeks take 34 hours to cook?
I don’t understand pre-cooked.
Are you cooking previously cooked meat?
Or did you sear or brown the meat first?
I don’t mean to be difficult, we just need more information on the Beef Cheeks to understand your problem.