Using `min_active_rowversion` for global temporary tables
The explanation would appear to be that the global temporary table is in fact in TempDB, not the database your code is running against. This is because the MIN_ACTIVE_ROWVERSION() function is scoped to the database level.

The min_active_rowversion() function seems to work when you use the following code

USE [db]

CREATE table ##mytable ( Id int, rv rowversion )

USE [tempdb]

select min_active_rowversion()

insert into ##mytable (Id) values (1)

select min_active_rowversion()

insert into ##mytable (Id) values (1)

select min_active_rowversion()


dbfiddle