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
Please see our announcement here.
My search through the commit log was looking for anything with filter, so I didn't see this one. However, this fix was applied to the master branch in Nov 2017 and so will presumably already be included in @stevelis' update from Github.There was a fix in GitHub which may address this issue
https://github.com/Fabrik/fabrik/commit/392b5a9c18f0db583933920ad281ab0e144c7826
seems this is creating a different "filter-grouped" string in the list paramsand the rest is in the JS file for the pre-filter tab on the back-end which handles adding new pre-filter rows,
yesAnd you will need to re-save the List as well I think.