This relates to JENKINS-59892, but I've managed to narrow it down.
The same job was run on two identical slaves. the job writes a property to a file, injects the file, and then attempts to echo it.
On one slave this succeeds, on another it fails. This is killing our system - many jobs fail due to missing attributes.
I've attached all the configuration files i could think of in the zip.

Note:
As a comparison, I also tried injecting a variable not from a file - that succeeds in both (see VAR_IN_FILE vs. VAR_IN_CONTENT in the injectedEnvVars files).

Additional info:
I tried using a full path (as opposed to a partial path) and it seems to work (e.g. $WORKSPACE/build.properties vs. build.properties).