Here's a 10 day trend of H1:SUS-MC1_BIO_M1_MON and H1:SUS-MC3_BIO_M1_MON, showing that MC3 BIO_MI_MON value has been at 1373 for 10 days, so no change in MC3 when MC1 started oscillating, but still a bit suspicious in that MC1 has an issue with an M1 coil driver, and MC3 has an unexpected monitor reading on one of it's M1 coil drivers.

Well, it turns out that this BIO readback problem has been going on forever since about 21:55 UTC on August 26 2019.

After: 1373 = binary 010101011101, bit 3 (4th significant bit)=1.

Before: 1365 = binary 010101010101, bit 3 is zero as it should be.

Bit 3 = 1 means that T3 coil analog input comes from the test input on the front panel, not from AI chassis. But this is clearly a readback-only problem because DAC output actually goes to the coil. https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=57144