2014 Kia Sorento with coolant issues

After some expert advice please.

I first noticed our kia losing coolant when I could see it coming down the right front wheel. Took it to our local mechanic who suggested maybe a head gasket issue and to take to our local Kia dealership (1 hour away). Before this, we took it to another mechanic in our town who did a pressure & combustion gas test and came back fine, but he replaced the radiator cap and said to just keep an eye on the coolant levels.

When cold - I opened up the radiator cap and it send bubbles into the coolant expansion tank - then the coolant level stayed higher in the expansion tank and didn't come down. The engine doesn't overheat and runs fine. Would this indicate a head gasket issue even though it's been pressure tested fine after the cap was replaced? or what potential other issues could cause this?

Many thanks, appreciate all your advice.