The default for the autogenerated JNLP container is to have it's image set to "jenkins/jnlp-slave:alpine" and the imagePullPolicy set to "IfNotExists". After some updates this might lead to very old cached versions of the image "jenkins/jnlp-slave:alpine" which might not work with the Jenkins itself anymore. This could be resolved by:
a) Setting the jnlp version to a compatible version on every new kubernetes plugin release (i.E. jenkins/jnlp-slave:3.23-1-alpine)

b) Setting the imagePullPolicy to Always (This might annoy some people because it will do unnecessary requests)

I've solved the problem for myself by creating a default pod template with the current jnlp-slave but that seems a bit like a workaround to me.