Usual approach is to grind finer which I’m sure you’ve tried what was the result? Are you using the setting that they have always used for espresso? If so, and they extracted at 25-30s then maybe they tamped overly hard to compensate for too large of a grind?
Need a bottomless portafilter to see if there are channeling issues.
So, one way of troubleshooting is to stop the shot at 40ml. Check the time and then taste. If bitter still, then 35ml. If not, then 45ml and so forth.
I would find the acceptable taste first, just for cafe service and then work on the problem separately.
Could also be a problem with the machine not generating enough pressure, nevermind what it reports as the pressure.
I wouldn’t get hung up on the color of yellow though, looking more for a transition change of color to a “thinner” color…
Many other people know more than me on this topic, I’m sure they will chime in to help.