Sure, sorry, I should have specified that I used the option to only allow specific dates using JavaScript using the following code:

const startOfMonthObj = LFForm.getFieldValues({fieldId: 44}).dateTimeObj;
const startOfMonth = dateFns.toDate(startOfMonthObj);

const endOfMonthObj = LFForm.getFieldValues({fieldId: 45}).dateTimeObj;
const endOfMonth = dateFns.toDate(endOfMonthObj);

if (dateFns.isDate(startOfMonth) === false || dateFns.isDate(endOfMonth) === false) return {
  allowed: false,
  errorMessage: 'Please select a month'
};
return date >= startOfMonth && date <= endOfMonth;

I am using Forms version 12.0.2503.10378

Also, to clarify, the dropdown is what the user sees on the UI, but the dropdown selection triggers some JS that populates two date fields with the start and end date of the selected month, which the above code uses.

I can reproduce your issue now; I think it's related with setting limit with javascript.

Would you mind change to use field rules instead of setting allow specific dates using JavaScript?

Since you already have fields for start date and end date, you can simply configure field rule like this:

That does work, so thank you for that. However, if the scenario were more complex and couldn't be achieved with field rules, is there a way to achieve this without using them? Is there a way with JavaScript for instance to set the default month on the date picker? 

One scenario I could think of is if I want to only allow a user to select dynamic specific dates on a calendar based on information loaded from an external table - I don't believe this could easily be done through field rules and would require JavaScript. Would there be a way to set that to say the month of the first loaded date? I wouldn't want to restrict selection to only that month, but I would want it to start at the beginning.

Thanks!