I made a little video clip that shows a Thor in HIP mode fighting against a Liberator without range upgrade and a Siegetank.
I will quickly explain what you are going to see and why this AI behavior probably makes Liberators in mech vs bio a lot more powerful than intended.
First you will see the Thor outranging the Liberator on attack-move, which is intended since the Liberator lacks the range upgrade.
Then i add a Siegetank and place it directly under the Liberator.
Now when the Thor will engage, he will be attracted by the tankfire, move into the liberation circle, but then dont actually kill the Siegetank, but will instead shoot the Liberator again.
Then at last i will show that if you manually target the Liberator with the Thor, he will act normal and stay out of the Liberator circle, which results in a better trade.
Thor in HIP mode will generally prioritize ground units over air as far as i know.
Instead of a Siegetank, you could also use some other units to bait the Thor into Liberator range.
So technically this is working as intended and not a bug (i suppose!?).
But the problem I'm seeing is that the Thor doesn't actually proceed to kill the
static tank, but instead just moves towards him and then shoots the Liberator again.
So in order for the Thor to be a counter to unupgraded Liberators in a realistic game scenario, the Thor user has to always manually target the Liberators, because even if you micro the Thor back, he will always repeat the process of "chasing" the Tank, just to stop and then attack the Liberator again.
It's just very inefficient unit behavior and i think if the SC2 developers could find a way to improve this kind of behavior, it would help a lot to make the Thor a more responsive and "stable" unit (besides the upcoming balance changes to the Thor).
*Edit: I just wanted to add that this behavior probably affects a lot of other matchups and situations as well, but TvT was simply the matchup where i noticed it the most as a player, because of how common it is to face air switches in the current TvT meta.