Numeric Range Slicer only use available values

Your YYYYMM should be a calculation in your Date table. See the following M code.

let
    Source = {Number.From(#date(2018,1,1))..Number.From(#date(2018,12,31))},
    #"Converted to Table" = Table.FromList(Source, Splitter.SplitByNothing(), null, null, ExtraValues.Error),
    #"Changed Type" = Table.TransformColumnTypes(#"Converted to Table",{{"Column1", type date}}),
    #"Renamed Columns" = Table.RenameColumns(#"Changed Type",{{"Column1", "Date"}}),
    #"Added Custom" = Table.AddColumn(#"Renamed Columns", "YYYYMM", each Date.Year([Date]) * 100 + Date.Month([Date]), Int64.Type)
in
    #"Added Custom"

That last row -

Date.Year([Date]) * 100 + Date.Month([Date]

as a column will create 201801, 201802, etc. It will never create a bogus YYYYMM like 201875 or 201899. 

Use that column as your slicer.