Safari browsers now has a feature called Intelligent Tracking Protection (ITP), released in late 2017.
ITP is intended to protect end-users from advertiser cookies but unfortunately it's also affect legitimate cookies being used to maintain session of your forms (which is needed when you have form with multiple page or having review page). Hence you're getting the blinking issue, particularly when the form is being embedded from different domain.
To avoid this issue, you'll need to make sure the form is embedded into the page having the same domain. If your machform is located in different domain, you'll need to move it to the same domain.
If your machform is located in the same domain already but the form still blinking, double check the form embed code again, particularly the domain part. In most cases, the form is embedded to www.example.com but the form itself is coming from example.com (without the "www").