RR-NDB
"The speed falling seems typical of stagnation point and not drain."
I agree. However, if the drain plugged first, the first invalid excursion of a/s would be an increase. If the aperture plugged up with the drain, stagnation, and "frozen" sense of "a/s" read while both holes remain plugged. If the drain unplugged before the Aperture, the ias would approach zero.
So, if these pitots plugged in this fashion, and close to the same time, the A/S would be reported as "accurate", and faster than she was actually flying.
Also, a rapid and consistent increase in a/s could trigger WindshearAlert. It could also trigger OverSpeed alert, etc. These artifacts are present in the ACARS.