BTW, during this trouble shoot, we looked at the IR beatnote spectrum between the Xend and the PSL.
It showed a set of sidebands at ~200kHz, which is the modulation frequency.
There was another eminent component present at ~30kHz.
I'm afraid that there is some feature like large servo bump, a mechanical resonance, or something else, at 30kHz.
We should check it. Probably it is my job.