Confirmed and fixed. v83 cannot represent 'seated' and 'climbing' at the same time - it is the same exclusivity that stops you attacking on a rope - and our chair is an independent sprite pinned wherever it was placed, which is why it hung on the rope while you kept sliding past it. Sitting down while on a ladder or rope is now refused (standing up is always allowed, so this can never trap you).

(On the dev branch - it will be in the next published build.)