I have an AC2889 Purifier model 2020
Both the App as the (action) Flow works perfectly.
Accordingly the Philips manual the device has 3 Purifier mode: General - Allergen and Bacteria (no manual mode)
But changing the Fan speed change the Purifier to a manual mode. That’s correct because you have manually change the device through changing the Fan Speed (manually on the device self or in the App or in a Flow)
So I have tested both my devices (AC2889 and AC3829, both 2020) and there are no problems.
Maybe the problem is only on a 2019 model.