Question about pressure and level in coolant

[KuaiThunder]

Hi guys,

I notice recently when my car is hot the coolant level in the bottle would drop from Max to Min and sometime even below Min. The coolant would stay at the dropped level even the car cooled down. But when I open the cap and release the pressure the coolant level will go back up to the initial leval (Max). I pretty sure it not behaving like this previously. I only notice it recently, when outside temp reach 34C, as I get the low coolant flashing warning. I run three times, run car hot and cool it back, same thing happen. I searched the forum and I can’t find people with similar situation. Anyone know what is happening here?

2001 S Tiptronic with "04" cap and no moisture issue

Thx