Leaking toilet seat when pink flush is used

Yes, stop using the pink. It’s not necessary and seems to cause algae build up. That is a serious suggestion, btw.

I suspect it is merely splash back from the flushing action and the pink makes it noticeable. Clear water would likely go unnoticed.

Is it perhaps because of 'too vigorously' flushing the loo rather than a leak?  I stopped using the pink years ago - because of the dreaded black streaks -  and just flush with clear water but have a diluted spray of Zoflora to sanitize the bowl.

Good point, Jill. Flushing the bowl as you would a conventional toilet will both cause splash back and fill the cassette quicker than you can blink. A more restrained approach is best.🚽😀

It certainly would not cause a problem on my vans as that is the ay that I drained the flush water at the end of a tour before the final empty of the trip

I'm struggling to think how water can get from the flush tank to where it leaks from under the seat, unless the 'jet' in the bowl is partially blocked/clogged causing it to spray upwards?  Clean inside the jet with a pipe cleaner/bit of wire or baby's bottle teat brush??

I third the advice to stop using pink (it may be a blob of black mould causing your issue?) - I've been using a capful of Zoflora (or a cheaper 'pound shop' version) in the flush tank for the last 2 years, it gives a nice odour the flush and is disinfectant so has stopped any mould growth in the tank.

You should be able to flush continuously without any leakage as EasyT says (I do the same to drain the flush tank 'in use' if I need to).