When exiting Argentina and entering Bolivia on La Quiaca/Villazón border I didn’t get the entry stamp on Bolivian side. That’s because the two countries have some type of tourism agreement. So technically Argentina grants you permission to enter Bolivia. It confused me especially because my Spanish is quite basic. I got 30 days tourist visa. Then when exiting Bolivia I received only exit stamp.
It’s important to keep a document given by Argentinian customs when leaving the country.