Var renamed but original name [incorrectly] used in code === 'not defined' error

[frudman]

When running code through minify (BABEL_ENV=prod npx babel --no-comments --out-file dst src), a local variable gets "reduced" (i.e. from navx to c) as expected but a later reference (within a string expression) uses the original name (i.e. navx) resulting in a 'not defined' error upon execution.

Input Code

(function() {

    const totItems = 71; // REQUIRED for error: if commented out, NO ERROR!!!

    var start = 1, navx = '';
    while (start < 71) {
        navx += 'a';
        start += 10;
    }

    const pages = 'b' + navx;

    console.log(pages);
})();

.babelrc Setup:

{
  "presets": ["env"],
  "env": {
    "prod": {
      "presets": ["minify"]
    }
  }
}

Minified code

Using BABEL_ENV=prod npx babel --out-file dst.js src.js to build it:

'use strict';(function(){for(var b=1,c='';71>b;)c+='a',b+=10;var a='search result pages: '+navx;console.log(a)})();

Actual Output

Using node dst to test run this code:

/home/freddy/enerx/dst.js:1
(function (exports, require, module, __filename, __dirname) { 'use strict';(function(){for(var b=1,c='';71>b;)c+='a',b+=10;var a='search result pages: '+navx;console.log(a)})();
                                                                                                                                                         ^

ReferenceError: navx is not defined
    at /home/freddy/enerx/dst.js:1:154
    at Object.<anonymous> (/home/freddy/enerx/dst.js:1:175)
    at Module._compile (internal/modules/cjs/loader.js:678:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:689:10)
    at Module.load (internal/modules/cjs/loader.js:589:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:528:12)
    at Function.Module._load (internal/modules/cjs/loader.js:520:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:719:10)
    at startup (internal/bootstrap/node.js:228:19)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:575:3)

Expected Output

Using node src to run:

search result pages: baaaaaaa

Details

Something interesting (?): if I remove the unused totItems declaration (definition? :-)), the code builds and runs properly.

Is this a hoisting issue? (not a clue: just said that to sound smart! :-))

MORE IMPORTANTLY: if I remove the enclosing IIFE (i.e. (function(){})()), the code also runs properly.

[frudman]

more observations:

• if I change the const totItems = 71; statement to var totItems = 71; everything works
• if I change it to let totItems = 71; it does NOT work

(the value is not important, just a leftover from cutting down real code to minimally needed to reproduce problem)

[frudman]

looking for a workaround (for our real code), I noticed that the babel REPL is working properly on the same test code. It seems to use Babel 6.26.0, same as us, but Env 1.6.2 (we have babel-preset-env v1.6.1). Not sure which or which version of minify they use on that repl site.

Included a capture of test: