Is there a policy on this site about “changing a question after receiving an answer”? Example: math.stackexchange.com/q/3914281/42969. OP asks for the validity of an inequality under some conditions. After receiving an answer with a counterexample, the conditions in the question are modified. This invalidates the answer, and confuses future readers of the thread.

I am not really sure to which extent it is a policy, but there are certainly several related discussion on meta.

Question edited to change meaning, Are questions necessarily “static”?, Editing a question after receiving one or more answers, Is that OK to modify a question significantly？, etc.

Looking at linked (or related) questions you can probably find more discussions which are close to this issue.

I did not check the specific case you linked to, I just looked a bit through meta on older threads.

@MartinSleziak Thank you. I have passed on those links, and OP has reverted the change.