I would guess that something somewhere is switching at 08:20 and 16:00 ... could be in the house or a neighbour or a street light ...
Thoughts - it is something that is electrically very noisy and at 8:20 AM when it turns off there is enough noise to totally fill the line for a very short period and sync is lost. As the noise source has, by then, disappeared you resync at 9dB. The 9dB has been set because of line issues.
At 16:20 the same device turns on and kills sync, then for the next few seconds it still creates a lot of noise which is when sync is being negotiated. It actually syncs at 9dB, but the noise then drops off a little resulting in the 13dB level ....
The overnight drift from 13 to 9 and back to 12 is quite normal - there is more background noise at night.
Run it for a few more days and see if it happens at the same time. On one day, try doing a manual resync at around 21:00
M H C
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