Custom form button issue

Site URL: https://www.eshertennis.co.uk/junior-coaching-programme/p/red-stage-ages-5-6

Hi - an issue has come up on my website the past few days and I can't get to the bottom of it.

The buttons on custom forms only appear on hover/rollover (see attached images/video). It is only the custom form buttons that are behaving this way (all other site buttons are appearing and behaving as expected).

This issue has only just appeared and none of our site admins appear to have made any changes recently. So I'm not sure if this is an issue with Squarespace or if my buttons have suddenly started inheriting a weird style from somewhere.

Is anyone else experiencing this? And does anyone have any suggested fixes?

Thank you!

I've at least temporarily fixed this issue by adding this code:
.lightbox-inner input.button.sqs-system-button.sqs-editable-button {
    color: #0096E5;
    border: 1px solid currentColor;
}

But it doesn't seem like I should need custom CSS to fix an issue that has only just appeared and is making it almost impossible for customers to buy our products. So if anyone does have suggestions for a real fix, I would still appreciate it.