Why does my Winsor & Newton Winton Oil Paint crack when I use a 0.5 mm sable brush on a primed linen panel?

I have been working on a small still‑life using Winsor & Newton Winton Oil Paint No. 3 (Ultramarine) on a linen panel that I primed with a traditional rabbit‑skin glue gesso (2‑coat, sanded to 120 grit). The brush I chose was a 0.5 mm sable sable brush (model S‑B500, purchased 2022). After the first layer dried (about 48 hours), I noticed a fine network of cracks forming in the paint film, especially where the brush strokes overlapped.

What I have tried so far:

• Reduced the pigment load by mixing the paint with a 1:1 ratio of linseed oil medium (Winsor & Newton Winton Medium No. 1).
• Applied a thin “fat over lean” layer, allowing each coat to dry for at least 72 hours before the next.
• Switched to a synthetic sable brush (model S‑B500S) for the second layer; the cracks persisted.

Possible culprits I am considering:

1. Excessive stiffness of the rabbit‑skin glue gesso – perhaps the surface is too rigid for the oil film to flex.
2. Brush hair tension – a 0.5 mm sable may be too fine, causing the paint to be spread too thinly and dry too quickly.
3. Pigment‑oil ratio – even with the medium, the ultramarine pigment might be too “dry” for this substrate.

Has anyone encountered a similar issue with Winton oils on linen? Are there documented interactions between rabbit‑skin glue gesso and fine sable brushes that could explain this? I would appreciate any references to technical papers or personal experience that can clarify whether the problem lies in the ground preparation, the brush choice, or the paint formulation.

Thank you for any insight.