Azure pipelines - Stages are skiped despite no conditions - azure-devops

Question

I'm trying to set up a pipeline in Azure.
Actual behavior
On pull-request a build validation triggers the pipeline and all stages and jobs are triggered.
After the merge, all jobs are skipped.
Expected behavior
After the merge of the pull request, I would expect stage Bto be triggered
Question
What am I missing so pipeline triggers correctly on merge?
azure.pipelines.yml
trigger:
branches:
include:
- master
stages:
- template: 'main.yml'
main.yml
stages:
- stage: 'A'
condition: startsWith(variables['build.reason'], 'PullRequest')
jobs:
- job: A
steps:
- script: echo A
- stage: 'B'
jobs:
- job: 'B'
steps:
- script: echo B

Answer 1

The trigger feature only works for the whole pipeline and not for an individual stage/job in the pipeline.
Normally, we use the different trigger types (CI/PR, resources) and filters (branches, paths, tags) to define when the pipeline should be triggered.
In the pipeline, we generally specify conditions to a stage, job or step to define when the this stage, job or step should be run or not. The conditions will be verified after the pipeline has been triggered.
To specify conditions, you can use one of the following ways:
use the condition key.
use the if expression.

Answer 2

I found the issue and applied suggestions from #bright-ran-msft
Stage A and B are implicitly linked. Since stage A was not triggered on the merge, stage B would not start either.
Solution
Instead of using condition, it is required to use if
Example
stages:
- ${{ if eq( variables['build.reason'], 'PullRequest') }}:
- stage: 'A'
jobs:
- job: A
steps:
- script: echo A
- stage: 'B'
jobs:
- job: 'B'
steps:
- script: echo B