One possible reason of ITF recent fragility in windy condition could be a degraded accuracy of ASC_PR_TY, due to a worsened balancing of PR MAR Z actuation. The oscillation of PR TY was the cause of drops in the sidebands, spoiling the locking error signals (fig 1).

The coupling from Z to TY is visible in the transfer function Sc_PR_MAR_TY_CORR over Sc_PR_MAR_Z_CORR and it has been corrected by changing the driving matrix. The improvement is visible in fig 2. The amplitude of the TF, 4.5e-4, corresponds to the required relative change in the driving coefficients.