I think it’s the lack of expansion as you press the figments in. They are a push fit and theCA is brittle . If you’re even slightly out pushing the tube will flex but the CA will not , hence end cracking.
A reamer and secure the fittings with a drop of CA perhaps but tubes MUST be scrupulously clean internal DAMHIKT 🫣
Im still learning with CA and like Phil much prefer melamine lacquer.
I apply 50% thinned first and leave overnight to penetrate and cure .
You do ( or rather I found I do ) need very good quality brushes to flow it on though. There is little as frustrating as the” set stray hair “ in the surface when you come back to assemble one 