That’s very interesting. I do have some experience with this and a theory.
I have found that lighter roasts have this problem more often than the more soluble roasts. Has your roaster changed their profiles or coffee recently?
I realise that this doesn’t account for the fact it happens on one group and not the other though. Can I ask if both groups are running pouring at the same time when this happens or is it just when a single group is pouring (is the pump pressure higher or lower when the puck stick happens).
Ok, here is my big dumb theory:
The role of pressure-
At high brew pressure, when the water hits the puck it tries to expand but under 9 bars of pressure it can’t and compresses. At the start of the extraction, most all of the coffee in the puck is bearing the load of the 9bars of water from the machine, but slowly coffee particulate in the puck (with the least amount of load placed on it from all sides) is evacuated and taken away. This leaves a fine lattice of still compressed coffee grinds that don’t really move and at their load bearing contact points minimally extract. The water then flows more freely through these eroded fine channels between the grinds in compression with lessening resistance through the puck.
If the brew pressure is too low, the puck never compresses and the coffee grinds don’t compress together to impede the flow.
If the pressure is high enough to just compress the puck gently together, then the grinds in the puck expand as the flow takes away coffee particulate. The remaining grinds expand into themselves and create a higher flow resistance.
The role of roast-
Additionally, the lighter the roast, the less readily the grinds take on water and because of that, at the end of the shot, are more lubricated with water.
So to put this all together, I’m guessing that the slightly higher pressure from brewing with a single head only would leave a more porous coffee lattice structure in the puck and combined with the hydrophobic nature a lighter roast, the puck would contain more residual water between grinds making it more slippery at the bottom and sides of the basket. When the puck finally expands hard into the shower screen when the pressure drops at the end of the shot, it is the roughest interface and gets stuck there.
To be honest, I could be wrong, this is just a wild guess. My apologies if this is not the forum for that. I’m sure some experimentation would get to the bottom of it!
I would be interested to know how you go.