Hi,
We have had portal forms used here for last 6 months and didn't see this issue.
A specific field is not visible ( only label is visible) on Initial load of the screen. Since its mandatory field, the form doesn't allow to submit.
However, after we notified this, if we refresh the page again as soon the page is loaded with fields, the specific filed is visible and we are able to proceed to submit after filling in. Looks like caching issue only for this specific field.