Disabled dates seem to be clickable

There have been a number of occassions when the customer has been able to select a date which is meant to be disabled. I am disabling all Saturdays and Sundays, and then have added the current UK bank holidays. See below:

```
disable: [
1, 7,
[2020, 3, 10],
[2020, 3, 13],
[2020, 4, 8],
[2020, 4, 25],
[2020, 7, 31],
[2020, 11, 25],
[2020, 11, 28],
[2021, 0, 1],
],
```

This seems to work fine for me, but there have been a number of occassions where one of these dates has actually been passed through to our CMS, meaning the user has somehow selected one of the disabled dates.

I have been through some pretty rigorous cross-browser / device testing to try and replicate this issue, but have not manage to myself. The correct dates are always disabled, and there doesn't seem to be any way of selecting them.

But obviously something is not right if some users are able to.

Any help on this issue would be greatly appreciated.