Yes the beer is flat or mostly flat after the foam dissipates. I think all the carbonation is in the head.
The carbonation comes out of solution in the form of a big foamy head.
If using ball lock: do you have the correct keg posts on? This sounds like a problem I had a while back where I had threaded an out post to the gas in.
I had one keg where the dip tube was almost too long. If I overtightened the fitting the tube would hit the bottom of the keg creating a thumb on the end of a garden hose situation, it would spray out. Once I loosened that fitting the problem disappeared.
Well I changed out the “line out” post and poppet and cleaned the dip tube with no change in foam.
Do you think the dip tube is to close to the bottom forcing the beer to squeeze through causing unnecessary turbulence ?
This happens when you are pouring too fast/too much pressure. You blow foam out of the tap and when it settles you have flat beer with a huge head.
On some of my kegs, the dip tube is bent to pick up beer from the middle of the keg. When tightening the post, the dip tube will often rotate. If it winds up pinned against the side of the keg, the opening gets restricted and the pour becomes all foam.