I am using this same filter() in Pro 3.0 with a service published to Server/Federated 10.9.1. My arcade expression works fine in Pro, but it fails in Field Maps and Web AppBuilder with the error on the filter() - Line 4: "Failed to query statistics". Are there any limitations to this, I wouldn't think so.
My Holidays table is just a table of holidays, querying of the date column.
var t30 = dateadd($feature.initialreport,42,'days')
var t = dateadd($feature.initialreport,0,'days')
//query my date field - dt - on my holidays table
var sqls = "dt BETWEEN @t and @t30";
var fs = count(Filter(FeatureSetbyName($datastore, "SUDOECMS.DBO.Holiday"), sqls))
var dayz = dateadd($feature.initialreport,fs + 42,'days')
return dayz;