Trello evidently had a bug (or rather, Safari 14.1 evidently has a bug which manifested on Trello) related to Intl.NumberFormat:
https://community.atlassian.com/t5/Trello-questions/Re-Trell...
Does the following work for you in Safari 14.1?
new Intl.NumberFormat('en-US', {notation: 'compact'}).format(1825602)
In Chrome this gives "1.8M"