This is occurring with a few separate form pages I have with PHP 7.3.13. For example I have a simple form myForm.php with say 15 fields using method="post" and action="myFormAction.php". All input fields have a unique name="(whatever)". When I submit myForm.php, *sometimes the first time through, myFormAction.php will not have any data in any of the $POST fields. I do a var_dump($POST) and it revealed array(0) as if nothing was sent/received. I hit Back, change nothing on the form, submit again, and it's all there with array(15).
Browser is IE 11 (work purposes). Just tested in Chrome and it submitted fine the first time. Minutes later, I hit back and submitted again and got array(0). So something is off with caching or a time-out perhaps? However I first discovered this issue even when the forms are being filled out and submitted only seconds from form-load, with simple text and no special chars other than perhaps a date.
Again, this is happening in another area of my application as well, though not every submission.