Grounded for commercial use perhaps.
Southwest could of wanted their fleet at various centralized locations to initiate software updates/engineering refinements.
American pilots are well aware of how to operate against the software glitches. Engine issues seem mechanical, which hasn’t been a primary issue before with the Max 8.
Does Bill Clinton provide your definition of 'primary'? Because it was problem with the Max which appeared early.
From May, 2017: "Boeing grounds 737 MAX planes over quality issue with engine"
https://www.seattletimes.com/business/boeing-aerospace/boeing-grounds-737-max-test-planes-over-quality-issue-with-engine/
The MAX 8 and MAX 9 have new, more powerful and more efficient engines.