Esprima minification garbles class definitions?

I have this source code

class Field {
  constructor(index, buttonsPerLine) {
    console.log("Constructor called");
  }
}

var f0=new Field(0,3);

When I upload it into the Bangle 2 emulator using the Web IDE, I get "Constructor called" on the console, as expected.

When I upload it with enabled Esprima minification, I get

Uncaught ReferenceError: "Field" is not defined
at line 1 col 61
...Constructor called')}}var f0=new Field(0,3)

When I shrink the source code to simply

class Field {
  constructor(index, buttonsPerLine) {
  }
}

var f0=new Field(0,3);

I see the following error message on uploading the minified code:

Uncaught ReferenceError: "Field" is not defined
at line 1 col 28
class a{constructor(a,b){}}var f0=new Field(0,3)

To me it seems that the Esprima minificator changes the name of the defined class from "Field" to "a" and then cannot find the class "Field" - which is not too surprising…

Is this expected behaviour? Am I doing something wrong? I'm quite new to Javascript programming but to me the code looks totally reasonable.

Any help appreciated!
Best regards,
Dirk