Soil stack filling up with water BUT Loo is emptying

[DIWHY]

Even though my loo still flushes and clears the soil stack is leaking at the lower joints.
From previous experience I know it is filled with water from somewhere that cannot get away fast enough (possibly shower drain) but how.

Is it possible that my neighbours underground soil links to mine before it goes to the inspection pit.

I only say this as last time I had similar and I rodded, the rods went way way past my soil and cleared a massive blockage stuffed with well known undesolvable wipes.

 

[ChrisR]

The pipes can join up in stupid places sometimes.

The previous time it was the main sewer which blocked? Presumably the cover isn't leaking so this time it's different, and hard to clear. Obviously try rodding away from the inpection chamber back up towards the house(s) but you won't get past the bend at the bottom of the soil stack.

If you have a shared drain put in before 1935 ish the council will clear it, but that sounds unlikely here.

We had trouble with our downstream neighbours putting loads of rice down the sink. It would float, form a crust and block from the upstream direction, leaving their drains clear!

 

[DIWHY]

Hi, the first blockage was somewhere in the pipe from the houses to the insp chamber and even then we had wipes (henkle we don't use) and Efflu pushed up our soil stack.
We are 2 unlinked semi's and I'm sure we share a pipe going to the chamber (cos there is only 1 teeing into the main chamber half pipe.)

The other semi is approx 1m higher than us.

When I rodded it didn't come out of my pipe it went at least 3m past.

I just can't work out how the block starts to happen with the odd too high flushed water rise in the pan and the odd too low water level after the flush, then goes o.k but you just know its due to happen.

I'm sure its linked to my neighbours activity but how?