If the rest of your buildings are running smoothly, I'd say the only common denominator is that TBT PM.Another idea is to send a "secret shopper" to go through the tenant onboarding process on these vacant units.Yet another is to see if you can reach out to your remaining tenants (under the guise of a customer satisfaction survey, for example) to see if they can shed some light on it.

What are the most common commercial properties types lenders lend on? 

That is a common issue.  

I did notice that when he analyzed a deal, he was able to click on an option to re-evaluate a deal as a different type of purchase, such as a BRRR deal, re-analyzed as a Rental Property.

Rather than prior liens I would shift my focus to research on how the issue of notice (or lack thereof) plays out in this scenario.

what specific information is included in the notice of tax sale to lien holders, just sale info, or amount to redeem also ?

From my Fair housing attorney:“Did you see that Saint Louis Park passed an ordinance that anyone buying a property and raising rents or giving notices that have an adverse effect on affordable rents – will have to pay cost and relocation of up to $4500 per tenant?”

Or at least a lot of notices of foreclosure.  

You will find that it's common on BP to recommend that you don't hang onto no-mortgage houses just because they cash flow nicely that way.

Winn,The tenant has until midnight of the 30th to return the keys, don’t cause yourself trouble by going into the unit now since she has paid and given you proper notice.