Compression leaking to valvetrain

Hi!

My 318 was completelly rebuilt a little over 1400 miles ago.
Heads were completelly done (stock specs), compression ratio around 9:1 (Sealed Power 526ap pistons and Mr. GAskets 1121g head gaskets).

One day it started making some noise on the drivers side valve train, I took the cover off and found there was compression leaking upwards through cilinder's number 5 intake valve.

Curious part is that the leak occurs when this valve OPENS for admission, not while on compression (what could suggest the valve wasn't properly sealing to the seat)...so I guess the compression is coming from cilinder number 7, wich is under compression at the time.

First thought was burnt headgasket, but I took the head off and couldn't find anything wrong...

Any guesses?

Posting some pics, I have a video but not sure how to post it...
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WhatsApp Image 2018-12-09 at 01.57.14.jpeg