Bernie likely would state that you are getting this symptom because you are getting better. You have cleared out more of the crud/toxins and thus you are “cleaner.” When this happens you get more symptoms typically. You body could’ve been more overloaded before and now its not.
The phlegm likely will only be temporary for a few months and then it should start to reduce from my experience.
It also could be related to inflammation.
Bernie likely has some supplements that would reduce phlegm production.