"Bumpy" Delay2 control in 1776's Multiplex Echo Machine

[Longer713]

Hello good people of this forum.

After building dozen of this circuits I recently started to band my head to wall because of this weird issue.
During testing I can clearly hear that while Delay1 control is super smooth and it affects sound as long you slowly turn the pot, Delay2 in too many builds now (atleast 3 times happened exact same issue) it feels like there are 2 "bump stops" in first half of pot range and then really nothing happens in second half of the turn.

At first I thought it might be bad brand new Alpha pot, so I wired new one on extended wires instead of soldering directly to PCB because maybe there were some tension during assembly that caused pot to go bad right away but that didn't helped. So guess what.. I put entirely whole NEW board with set of new pots and PT2399s.

Still the same problem.
Any idea what might cause lack of smoothness in Delay2 pot?

I highly appreciate your help in that matter ;_;