Examiner proof-reading master thesis before submission. Is this allowed?
Whether it is allowed depends on the specific regulations of your university, but your advisor proofreading your thesis and offering suggestions is quite common. I would consider not having a proofreading round to be lazy on the part of the advisor.
Keep in mind that the examiner can still have the initial reading influence the grade. Somebody who submits a perfect thesis that requires no corrections would usually get a higher grade than somebody who submits the same thesis after receiving detailed comments on how to improve it. The proofreading is part of the teaching process, and how well you implement the suggestions is part of the assessment.
What do you think is better from a didactical perspective?

Telling a student that they got a barely-passing grade because X and Y need to be improved

Telling a student that they could improve X and Y by doing Z, and having them do that before assigning a final grade


Clearly, the student will learn much more from option (2).