application.properties always assigned to default value

Question

For context, here I'm using intellij and Spring boot 2.0.0.

I have this configuration in my spring boot project's application.properties

prop=${PROP_VARIABLE:defaultValue}

Then I assign someValue to PROP_VARIABLE in intellij's gradle task run configuration. When I check the env via actuator the prop is always assigned to defaultValue, but if I assign it like this

prop=someValue

It's correctly assigned to someValue

Where's the problem here and what can I do to inject the env properly?