There is no performance issue with UDDTs .
There is no Warning message when UDDTs is use in Predicate.
select * from sys.columns
In this query notice the system_type_id & user_type_id.Sql optmizer already know and uses system Data Type.
I can't reproduce any warnings or performance differences in this scenario, even when joining bVarchar to varchar(10). EG
create database tt
alter database tt collate Latin1_General_BIN
create type bVarchar from varchar(10)
create table a(id int primary key identity, b bVarchar, c varchar(10));
create table b(id int primary key identity, b ...
This is not a collation issue. This is a language / locale / culture issue. Your DB (or your login's default language) is set to "French" (most likely) and they use Day then Month formats, not Month then Day, hence your statement is attempting to use month 13.
Try this to reproduce:
SET LANGUAGE French;
DECLARE @datevar DATETIME = '2020-10-13 04:42:...