Why isn't Jenkins merging a commit?

Question

git

We have an account in Bitbucket Cloud, a server with Jenkins. I set up the Jenkins + BB connection, also installed the pull request plugin, the task is to test PR before merging in master. Everything seems to be nothing BUT, sometimes for some unknown reason, Jenkins writes that everything is buzzing when receiving changes from the repository, writes that the commit has merged, and fails the tests, because we have
1. PR in which there are changes with errors
2. Tests fail - that's all logical
3. We have a PR update in the update and the test should pass
4. The test fails.
I check the Jenkins console log - there are no errors. This nonsense has happened several times already. I can not understand where to dig and where to look for the problem.
Since I’m making a commit in PR that has an error - Jenkins crashes the task on a merge conflict - that is, the merge seems to work, but it seems that it doesn’t always ... crawl and merge a commit to master after which the tests pass ...
I also noticed that Jenkins writes to me

git merge --ff cc0998bfde8a85e4febd663fb93ce15e95158da9 # timeout=10

that is, in fact, I have to

git log -n1

see a commit with this merge, but it is not there. In my master, the last commit is the one that should be in the master, but there is no one that needs to be tested. But there are no errors in the Jenkins log and it says that the merge passed....