Why does my gesso keep bubbling on a 30x40 cm oil canvas (Winsor & Newton Winton #712) after 24h?

so I've been grinding on this new piece for the past week. 30x40 cm stretched cotton, primed with two coats of Winsor & Newton Winton gesso #712, each thinned 1:1 with distilled water.

Everything looked fine until last night the whole surface started to bubble in patches—like tiny fish‑scales. I thought maybe the humidity, but the studio was at 45% RH, same as usual.

Things I've tried:

• re‑sanded the bumps, re‑applied a thin coat, let it dry 48h (still bubbles)
• switched to a faster‑drying acrylic gesso (nope, same story)
• checked the canvas tension, it's still tight, no sag.

My palette: Winsor & Newton Winton 712 oil colors, linseed oil medium 3:1 lean/fat, no alkyds. Underpainting was a quick grisaille with a thin glaze of burnt sienna.

Any guesses? Could the pigment in the gesso be reacting with the linseed oil bleed? Or is it a batch issue? If anyone's run into this with the same Winton #712, would love a heads‑up. Also, any quick fix before I have to scrap the whole thing?

(Attached a pic of the bubbling area – the texture is weird, looks like a micro‑crack network.)