Answer credit to @AlexeyBelkov – Answered here: https://youtrack.jetbrains.com/issue/KTIJ-893
The syntax highlighting feature worked after:
- Delete
~/.gradle/caches - Delete
~/Library/Application\ Support/Library/JetBrains/IntelliJIdea2020.3(or~/.cache/JetBrains/IntelliJIdea2020.3for ubuntu/linux users) - Delete
<project dir>/.gradle - Delete
<project dir>/.idea - Start IDEA and reimport the project.