That type of fitting seals on the flange-face, not via the threads. You need a sealing washer such as a soft copper, fibre, retained o-ring or Dowty type device to effect an engineered fix.
The other 3 joins dont have the washer you are describing and seem fine. I'm inclined to PTFE it up again and apply LSX. I do think this join has slightly less PTFE tape than the joint that is now fixed
Update: I pulled apart the final leaking joint. Added more PTFE tape and LSX. I then left it to dry overnight and have now had the water running at full pressure for over 3 hours without any leak. I think it's finally resolved although I have noticed a small smear of water appear after several hours in previous instances. My paranoia I think...
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