I would suspect this to be early manifestation of joint bond degradation between the aluminum and carbon, usually some sort of epoxy.
It is mostly a surface phenomenon hence the bubbling and likely the epoxy inside the joint is intact. Aluminum oxidation is possible
but I think the epoxy is a more likely explanation. Sanding down the area would help delineate this but can be deferred. This is
almost certainly not a butt joint but a slip joint with some tubing inside the other and epoxy as the interface. When the paint begins
to pop off you can investigate further but I would keep riding.

You can google aluminum corrosion and get various opinions eg: https://www.proboat.com/2014/12/keep...t-on-aluminum/
This kind of cosmetic joint degradation was common on Trek bikes in the '90s when straight carbon tubes were bonded to cast aluminum
lugs mostly clear coated rather than painted.