intermittent recaptcha fail

We have Marketo forms embedded on a WordPress site. The forms have recaptcha turned on.  Lately, every so often, a form fails to submit and we get a "Submission failed, please try again later." message. When I look into it, I see a "errorCode 400, Captcha field missing in form".

We do *not* have WP Rocket or any other optimizer installed, so, there are not those settings to change that I have seen in other threads. (https://nation.marketo.com/t5/product-discussions/can-t-submit-the-form-with-recaptcha-enabled/m-p/332242#M187700)

It is very random, I can sit there and submit tests all day and not see the error pop up, then suddenly it will.

It has also been reported by customers, so, thats causing some worry with the marketing team.

This is the form - https://us.sios.com/demo-request/

I'm out of ideas - other than starting to turn things on and off on the WordPress side - but - since the error is so hard to repeat, that could take a while to get sorted out, if we can even tell. 

Any other suggestions are welcome - thanks in advance.