Hi Lawrence,
I'm having this same issue on my Kentico 10 page using the "Simple CAPTCHA" form control. It was fine for a bit, now the image fails randomly. There are no JS errors in the console.
I see that you switched to an Invisible ReCAPTCHA on your form: are you using a Kentico extension for that? If you did it manually, how did you implement the server-side part of it (assuming you're using Kentico Forms)?