After a failure of lock acquisition at 12:35 UTC on December 10, both the green and IR flashes at the X arm suddenly disappeared.
I identified the cause as EX, because the Y-arm flashes were healthy and the MICH interference also appeared normal, indicating that EY, IY, and IX had no issues.

My current understanding is that the EX suspension may be touching something, particularly in the yaw direction.
The EX yaw response to changes in the optic alignment value is very slow or otherwise abnormal.
When I set a smaller value (e.g., 20) for the EX yaw optic alignment while the recorded value was approximately 50, EX moved as if it were coming off a constraint, and I could observe flashes at certain points.
When I restored the recorded value, I sometimes observed continuous flashing (but the flashing disappered soon) and sometimes did not (Fig. 1).

Further investigation is required.
This issue can be compensated by tuning of the upper stage of the suspension.