For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
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It works for me in the list view with the same settings, so the output is $2,300.00
Do you have your screenshot from details view? In details view (when element is read-only) it seems that thousand separator and decimal point are not considered at all, regardless whether you have something in "Format string" field or not.
To format the number in details view you could use some jQuery in details_xx.js and in "document ready" function loop through all the "number" fields with "each" function and format them as you like. As this is pure jQuery, you will find lots of examples for that in Stackoverflow.
I guess you could call it a bug. I cannot see any solution except the one I wrote above without hacking the core code.