gslb prober pool round robin mechanism

Here is the only thing I was able to find out:

https://my.f5.com/manage/s/article/K000133566

Sounds like you may want to enable debug mode (https://my.f5.com/manage/s/article/K19451442) and observe what is happening. Your question is theoretical and may not be easily answered here, nor if you submit an F5 support case (this is like an engineering or product manager type of question). However, these resources shared here should give you enough tools to test and figure out the answer on your own accord.