The UAT air bubble should not change in flight unless you have a leak.

I use a felt clunk in the fuel tank of a Wren 44 which I run without a UAT. I have found that the clunk itself generates cavitation in the fuel at high throttle settings with a high fuel flow, causing a line of tiny bubles in the fuel line. The engine can handle these SMALL bubbles without any issues.

Are you using a filter type of clunk in the main fuel tank? If you have searched for leaks everywhere & have found nothing perhaps the clunk is generating these tiny bubbles which gather in the UAT. Check the fuel line from the main tank to the UAT for these tiny bubbles. - John.