If fields 422 and 423 are populated by lookup, then I'm guessing your code is running before the lookups are complete.

I just tested this code on Version 11.0.2311.50556 and it worked for me (note that I believe the LFForm.changeFieldOptions functionality was introduced in that version, so it won't work in a lower version).

//Trigger the PopulateDropdowns function when the form loads
//or either of the trigger fields are updated.
PopulateDropdowns();
LFForm.onFieldChange(PopulateDropdowns, {fieldId: 423});
LFForm.onFieldChange(PopulateDropdowns, {fieldId: 422});

//Function that populates the drop down values based on a 
//range of values from two other fields.
function PopulateDropdowns() {
  let minUnits = parseInt(LFForm.getFieldValues({fieldId: 423}));
  let maxUnits = parseInt(LFForm.getFieldValues({fieldId: 422}));
  let newValues = [];
  for (let x = minUnits; x <= maxUnits; x++) {
    newValues.push({label: "Choice " + x});
  }
  LFForm.changeFieldOptions({fieldId: 438}, newValues, "replace");
}

I've wrapped everything in a single function, so it's easy to call three different ways (when the form loads, and whenever one of the two fields are changed).

I added parseInt functions to ensure that it properly handles the values from the lookups as integer values - just in case.

I moved the LFForm.changeFieldOptions out of the loop and did it as a replace instead of an add.  This is so that it can be run multiple times and always replace the full list instead of assuming we are starting with an empty list and only adding the select values (see above where there are multiple options to call the function).  This is facilitated by creating an array of values and then adding the array into the drop-down instead of doing one-off adds.  This also helps to ensure your list of values is in an expected order, since we're only doing one addition of the list instead of making a bunch of asynchronous calls adding items one at a time.