Jenkins sometimes a little bit strange but it works at is.

you have addes a new ssh key and you got following error “you have requested strict checking”

Under configuration “Global Security” scroll down and choose what you need

look a the screen snipet, this config works well.

https://docs.cloudbees.com/docs/cloudbees-ci-kb/latest/client-and-managed-masters/host-key-verification-for-ssh-agents

https://issues.jenkins.io/browse/JENKINS-43062