I am not a phonak expert, but those are my thoughts.
If speech gets muffled in loud situations, this can be
a) The automatic switching to some other program (which is what you guess)
b) the compression
With phonak Audeo V90, the standard program (number 1) is always the flow-program. This can have several sub-programs, and each of those can be set up individually.
a) can be ruled out if you copy the “normal” sub-program of the flow program to program slot 2. If you use program 2 and the problem is gone, it is shown that the automatic switching causes the problem. If not, consider b)
b) Compression means that loud sounds are amplified less than soft sounds. This is a good thing most of the time, because then you can understand soft speech without the loud noises blowing off your ears. But if you come to a noisy room, the compression regulates the aid down and speech becomes less clear. Also, many aids have a slow compression and a fast one. The fast one reacts immediately if a loud sound comes into the aid and thus protects you from loud sounds. The slow one reacts if a loud environment stays loud for a longer time (some seconds at least). It then regulates down the whole amplification and stays like that for some time even when the noise is no longer present. The problem is that you need to change the compression behaviour in loud environments, because the speech-area should not be compressed too much. Soft speech is around 50 dB, normal 65 dB, but in loud environments, people talk louder. So the area where speech stays uncompressed must be shifted up in loud environments. What I am trying to say: It is not necessarily a bad thing if the aid reacts to the loud environment and changes the compression. But it is likely that your aid is not set up correctly to do so.
When I looked over my audi´s shoulder, I saw that Phonak aids with all those endless features are really difficult to set up. My aids (bernafon Juna) have much less features, and thus are much easier to program. There have been other people here who complained about the sound-changing of Phonak Audeo V90, whereas others really love them. So my conclusion is that they are hard to set up.
Be patient, It´s probably not impossible !