Paper currency got its start as little promissory notes to pay gold, and then because the notes were more portable than gold, people started using the notes as money instead of the gold itself, which is why you sometimes see references to "one dollar notes", etc.A seller may or may not accept a promissory note as payment. 

Does this not work for properties bought with foreign currency?

Started back in 2010 when the US dollar started to get weak and foreign currency was gaining some strength.

Fortunately for us most of that crowd is into crypto currencies these days. 

And even with just cause you need to(waive last month for ex) pay for one month of their rent or provide them with one month of market rent funds upon move out.

If currency goes to heck - and the new currency is marbles - I feel someone, will pay me marbles to live in my dwelling.  

Less currency risk and conversion costs.

In short order you are paying off dollar notes now worth 75 cents with inflated fiat currency now worth $1.50.

For ex: If I get a HELOC on my primary home in January 2022, but like 6 months later turn that primary home into a rental and purchase a second primary home, would this be an issue?

Barbara,There is a lot to consider on Purchasing an Airbnb like this, for ex: new laws/rule, the cost/expense to run, cost of amenities, employees, cost for maintenance and repairs, insurance.