With 0 you only disable subscription you don't remove it.

Test it with gtm25 that have max 20 subscription if i remember correct.
When you fill all 20 then try with last byte 0 for 1 subscription and you will see that you can't add new subscription becouse you only disable it ( hide from garmin ) and you still have 20 subscription not 19 like you think when you say that 0 remove subscription.