Configuring PyCharm for ES6

I have two Vue projects in PyCharm, both configured identically as far as I can tell.

Both are configured to use ES6 under Languages & Frameworks -> JavaScript.

In one of them, code completion and linting for ES6 code seems to be working exactly as expected, the other one seems to be stuck in ES5 (or earlier?) mode, as it is marking all ES6 features as errors.

Has anyone had any trouble getting PyCharm to correctly lint ES6 code in some configurations?

Are there any magical options that cause PyCharm not to honor the configured language level in Languages & Frameworks -> JavaScript?

Kind Regards,

Michael Thomas