How to reproduce:

Use multibranch-pipeline workflow with merge PR strategy

1. Create a branch, develop it and make it nonmergable into the target branch
2. Push the branch and open the Pull Request in GH
3. Find out that the branch is not-mergeable
4. Fix the code, push the newer revision

Expected Result:

The pipeline is triggered.

Actual Result:

1. On step #2 Jenkins gets the 404 in the request to ...refs/pull/<id>/merge/contents
2. The 404 is cached
3. On the step 4 it does not query GH but rather returns a cryptic error of 'Jenkins File not found' although it does not check for it at all

Proposed solution: do not check for the contents of the "refs./..../merge" URL if the PR is not mergable.

Versions:

Jenkins 2.190.1
GitHub Branch Source Plugin 2.5.8+ (some irrelevant local patches)

Here is how we figured out what was happening:

1. Get the repro of the bug
2. Jump to the jenkins master node and create a copy of github cache dir, namely "${JENKINS_HOME}/org.jenkinsci.plugins.github_branch_source.GitHubSCMProbe.cache"
3. grep for the pull request id in the *.0 files to figure out the  request id (prefix before the '.0') of the exchange with Github 
4. zcat the file of the exchange (<reqid>.1) and see that Jenkins checks the refs/pull/<prid>/merge/contents and gets 404
5. Jenkins never consults anything after getting this response until the cache entry with 404 is purged, but writes a cryptic message like 'Jenkinsfile not found' and (possibly) the lines saying that the PR branch is dead, thus being removed from the list of branches to run the build against,

While surfing looking for the similar bugs, I noticed several that might be related to those ones, especially the first one.

https://issues.jenkins-ci.org/browse/JENKINS-48571

https://issues.jenkins-ci.org/browse/JENKINS-46290

https://issues.jenkins-ci.org/browse/JENKINS-54126

https://issues.jenkins-ci.org/browse/JENKINS-57206

https://issues.jenkins-ci.org/browse/JENKINS-57411