shower drain sinking on one side

I've noticed for a while that my shower drain is sinking from one side (attached). I have called a plumber to check and they have confirmed via thermal cameras that no water is leaking through the waterproofing into the sub floor. Based on that the have advised me not to do anything.

I want to sell my place but am concerned that the look will put some buyers off / a building report will classify this as a major structural defect.

Is there an easy way to make this look better? is this a major defect?