Rear Specialized Purgatory keeps developing an oily-ish sheen on both sidewalls, generally following the direction of the layers in the sidewall’s construction.

I’ve ruled out over ambitious chain oiling, disc brake leaks or an overly excited exploding Reverb.

Tyres are tubeless, but front Butcher is unaffected.

Has anyone come across this before?

It’ll be a bit of sealant weeping through.