i also had HVAC mystery issues in my first duplex when we house hacked it.Regarding the Daikin Mini Split - I have one in the 3rd story of my current home and it seems to work great for us. 

I would love advice from the community on this issue.  

I want to share my experience with Jenny Yi, an STR designer in case others have had similar issues.

Hi first post here just looking for advice, I rent a multi family home 2 unit for about 15 years have been lucky to have both units rented for 10 years no issues, well until now.

Can anyone recommend good companies to root cause  and implement solution to fix this issue?

They may also be able to temporarily relocate your tenant to avoid any other issues.

I had a similr issue a few years ago with a property in Cleveland where my property manager hit the same roadblock.

She walked away with $1,500 in hand, and we salvaged the deal, netting $8,500 in the process.That deal reinforced two crucial lessons: first, title issues can create unexpected hurdles, even in straightforward transactions.

Find a lawyer and issue a lease violation for other issues such as excessive noise and furniture removal.

Class midwest issues.