2015 Xc60 D5 coolant behaviour

[Ross1990]

Good morning, I am after some advice from the D5 guru’s.
My car is a 2015 xc60 D5 AWD With 90k Full Volvo service history. Had radiator replaced nearly 1 year ago due to common small leak from lower corner.
Since then I have been having some weird coolant levels.
So starting from topped up to MAX when cold every 6-8 weeks (2000 miles) the coolant level will go from Max cold to just above the MIN cold.
If I unscrew the reservoir cap after its been sat all night I get a slight hiss and the level will slightly rise, but not always.
No leaks anywhere everything is bone dry.

Question is do these systems take a while to fully bleed out when you have done a radiator change (possible air pockets still in the system?)
Car runs absolutely perfectly no overheating or anything. Just something I have noticed over the past 6 months.
Thanks in advance.
Ross