I think it’s actually related to the jQuery library update bundled with WP 5.6. The new version deprecated some string functions which were used when determining the map center. I put in a workaround for the missing functions, which works for strings – but it failed in the case where the center is saved as a lat,lng.
I’ve just published 2.58.1, which should fix this error. Please let me know how it works for you!