Should be expected, when probing bed, z axis moves up and down quickly which caused binding.
If you were able to only print in vase mode on a perfectly flat surface, you can get away with no binding issues. But to allow proper bed probing and z motion (z-hop, etc.), a second belt was added to support the opposite side of the bed.
Z-axis is stiff when attempting to rotate in the y direction.
Also very stiff when pushed in the Z direction on the supported side (held by the belt and stepper). However, the non-supported side does have flex. Its essentially acting as a cantilever bed as the carriage isn't fully constraining that end.
With about 10lb of force applied, Z on unsupported side will move up and down about 0.9mm.
- measure flex with unsupported side carriage removed (to simulate a cantilever bed). Another option might be to put dual carriages on the supported Z linear rail and use triangular support of the bed.
- extend 2020 to meet face of L bracket, see if it provides additional stiffness