Issue might be in your third line for the filter. You have it filtering to 'gravesiteid = @gravesiteID'
I think it should be 'gravesiteid=' + gravesiteID.
Here is the code I am using that is working. In the featuresetbyid I am adding a third parameter to just grab the specific field.
var join_ID = $feature.ID
var fs_prevprojyear = FeatureSetById($map, "18cf513418f-layer-3", ['Project_Ye'])
var filter_relate_feature = First(Filter(fs_prevprojyear, 'ID=' + join_ID))
return filter_relate_feature.Project_Ye