Hello. We’ve looked into this, and it is a bug, sort of.
The way our system for granting Shadowlands Beta access works is: it flags the Battle.net account, and at the same time flags the whole account for Shadowlands Beta forum access.
Months ago, we had an issue where those who were flagged for the Shadowlands Beta weren’t receiving their forum access. In order to resolve that issue, we hardwired forum access to a marker on the account that says “this is a Beta account”.
The side effect of this was: any account that had Beta access in the last two Betas (Legion or Battle for Azeroth) is now able to post in these Shadowlands Beta forums.
We’re looking for a more elegant solution, and we apologize for the confusion this has caused.
In any case, if you have been flagged for Shadowlands Beta access, you will see Shadowlands Beta in the In Development section of the Version selector in your Battle.net Desktop App.