Incorrect GitVersion config results in odd versioning

I've a Solution that invokes GitVersion in Azure DevOps.

The repo was last tagged "1.4.1".

GitVersion.yaml reads

mode: Mainline
branches:
  feature:
    increment: Minor
  hotfix:
    increment: Patch

The last master build resulted in build 1.4.1 as expected.

I created a new feature branch from master (feature/My_BranchName) and that resulted in build 1.5.0-My_BranchName.1. But when I create a pull request, the resulting build is 1.4.2-PullRequest1234.1 merging it to master results in 1.4.3.

I was expecting the PR to generate 1.5.0-PullRequest1234.1 and the merge 1.5.0

I can't figure out why I'm getting the results I am and would appreciate a pointer or an explanation of why it's doing what it's doing.

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