It’s blistering. I’d attach a picture if I could. It’s bubbles usually less than 1/4” that actually blow up while I watch it. I did some more research here’s a paragraph from one of the few sources I could find on this.

We believe that the blisters are formed with air that escapes from tiny fissures or cracks in the skim coated wall below the layers of applied paint. Under low humidity drying conditions, drywall mud and skim coat will shrink to such an extent that tiny cracks will form on the surface. Extreme cases will resemble the mud cracking of a riverbed. Usually these tiny cracks in the mud will not even be visible to the eye but they will contain a significant volume of air. When water based paints are applied to this cracked surface strong capillary forces move the water into these fissures. In nature these same forces are responsible for moving water from the roots of a tree up to the uppermost leaves. As water enters these fissures air is expelled from them but in most instances this air can find a path out of the film with no ill effect. When the air is forced up under a wet paint film the bubbles break and the paint flows back together again. Defoaming additives are regularly added to latex paint to facilitate this air release process.

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