Jenkins Groovy pipeline #tmp dir() AccessDenied exception for createDirectory - jenkins

Question

I'm trying to set-up a simple (single) Groovy pipeline script that would deploy my PHP project using webhooks. Sounds like a simple script and I've had it working in an older Jenkins version (which used bash script) but when I use the dir(String) {} method I get the following exception:
java.nio.file.AccessDeniedException: /srv/www/staging#tmp
at sun.nio.fs.UnixException.translateToIOException(UnixException.java:84)
at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:102)
at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:107)
at sun.nio.fs.UnixFileSystemProvider.createDirectory(UnixFileSystemProvider.java:384)
at java.nio.file.Files.createDirectory(Files.java:674)
at java.nio.file.Files.createAndCheckIsDirectory(Files.java:781)
at java.nio.file.Files.createDirectories(Files.java:767)
at hudson.FilePath.mkdirs(FilePath.java:3563)
The server itself runs on CentOS 7 and Jenkins is using version 2.303.3. I've added the jenkins user to the apache group, which is the owner+group of the relative webroot, located in /srv/www/staging. The permissions for the /srv/www and /srv/www/staging directories are set to 0773 apache:apache.
When I simulate a bash session with user jenkins using su -s /bin/bash jenkins, I am able to create directories and files within the /srv/www/staging directory - without any issue. So I'm wondering how the pipeline itself doesn't have permissions to do so!..
The pipeline script is as follows:
pipeline {
agent any
stages {
stage('Git Pull') {
steps {
sh "whoami"
dir ('/srv/www/staging') {
sh "pwd"
}
}
}
}
}
The sh "whoami" returns jenkins, so I'm sure the proper user is executing the commands.
I can't really find a descriptive similar issue, thus why I'm posting this.
Hopefully someone is able to guide me in the right direction.