What’s their rationale? Maybe it’s time to get a better trainer?

If anything when trying to gain muscle the recommendation is to lower the protein to ~0.8grams/lb of BW in order to accommodate for more calories usually carbs since protein is more satiating. This is assuming you’re eating a mixed diet of course.

For most people it doesn’t make a big difference either way, but if you’re struggling to gain muscle you would likely be better off increasing carbs not protein, or fat if you’re doing keto or carnivore.