Leaking flush pipe - my flush pipe stays full of water?

I have recently had a new BTW pan installed (separate concealed cistern). However, the flush pipe and elbow joint at the bottom of the cistern stay completely full of water after flushing and the joint to the toilet (not the cistern) leaks gradually all the time (not just when flushed). I've had 2 different plumbers and 1 engineer from the manufacturer look at this & have had 3 different pans (as manufacturing defect originally suspected) but none of them have solved this problem. They all tell me that the flush pipe should not stay full of water but no one has been able to solve this problem. The flush pipe is the correct length, it's at the right height / angle and has proper rubber connector.

If you have any suggestions on what could be causing this / how I can fix this problem I would be really grateful to hear from you.

Thanks.