on prem Azure DevOps Server 2020- PRs cancelling in process Build Validation builds

Hello-

We have an "on prem" Azure DevOps Server 2020.  For several of our branches, we have a branch policy requiring a "Build Validation" gated check-in build.  We have a build definition with one build agent for this.

Our issue is that if someone submits a new Pull Request and another person has a Build Validation build going from another PR, the first build is automatically cancelled mid process and the new PR's build starts.  This is very frustrating of course- and require some management of PR builds which should not be needed. 

Does anyone know how to stop this behavior of a PR cancelling any in process builds?

Many thanks!