Release version shows as unknown on deployment preview in Bamboo Data Center
Platform Notice: Data Center - This article applies to Atlassian products on the Data Center platform.
Note that this knowledge base article was created for the Data Center version of the product. Data Center knowledge base articles for non-Data Center-specific features may also work for Server versions of the product, however they have not been tested. Support for Server* products ended on February 15th 2024. If you are running a Server product, you can visit the Atlassian Server end of support announcement to review your migration options.
*Except Fisheye and Crucible
Summary
This article explains why release version has an unknown tag associated to it during the deployment, refer the image below : 
Environment
This issue is applicable for any Bamboo version >=9.4.x
Solution
In Bamboo 9.4 release there was a new feature introduced to Safeguard your deployment environments against unapproved releases.
To help you make sure that a bad release never takes down your whole production environment (or never does that again), deployment environments now let you set their release approval policy. The policy lets you define the conditions a release must meet before it can be deployed. That is, which environments accept all releases, which ones accept approved releases, and which ones only accept releases that haven’t been marked as broken. If you change your mind later, just update the environment’s settings and you’re good to go.
Learn more about the release approval policy for deployment environments
Because of this, the releases have now got a extra tag associated with them. UNKNOWN basically means that the release is neither approved nor broken, see example below
Case 1:
Here release 1 is showing as UNKNOWN because I have neither approved it nor made in broken but I have deployed it to some environment, see below.
Case 2:
Here release 2 is showing as Approved because I have approved it's status after it was deployed.