Had same problem. Leak went through the floor to our dining room. It's a bad design. Rubber O-ring appeared to go hard and loose it's elasticity. They should have learned from the space shuttle booster disaster
They say in the instructions the flush pipe must be supported so that when you push the WC pan onto the pipe no load is transferred to this joint. I don't think that's enough to prevent it leaking.
Replacement O-rings were on sale on ebay but I noticed the seller had sold hundreds so this must be a common fault.
Consequences of a major leak at this point can be dire as when the water levels fall the cistern fill valve will open. Nothing to stop the whole reservoir leaking into your house.
Mine was so loose that if I laid the cistern down the weight of the L pipe was enough to make it rotate. Got a video of that somewhere.
I replaced whole cistern as I didn't want to have to retile my bathroom again to get access.