Previously working installation now throws error when attempting to make a donation

I've just discovered that this module has stopped working for me.

It had been working okay, configured for both single and recurring donations.

Now, when I attempt to make a donation it throws an error message on the page saying:

2 errors have been found: AmountCustom amount
1 error has been found: Custom amount

If I switch to having just the one donation type (single or recurring) and disable custom amount, I get the error:

1 error has been found: Custom amount

This is even though the form does not present custom amount as an option.

There are no notices in Watchdog.

No changes have been made to the module code.

I'm running Drupal 8.9.12.

I don't know when this issue first appeared, and can only assume that it's been triggered by a change somewhere in Drupal core which is possibly causing this module to behave as if fields that should be hidden are now required.

Is anybody else seeing this same issue?

Does anybody have any suggestions for a possible resolution?

Thanks