Hey Rui,

I thought I found a workaround but it is not holding up long term.
We are on 10.4, and so far it's seen on some older Android tablets, iPhone X, and iPad mini among others.
But it's a bit more complicated; we are housing this form in an iframe and the iframe is influencing the dimensions of the form at the time it loads. So it registers as loading on a mobile device, but the dimensions are off.
The result is that some devices load the signature pad like that.
The workaround I'm using is hilarious and ugly and just replaces the css so you can continue to use it:
 

/****************************************************/
/****************************************************/
  let fixArr = [];
  $(document).ready(function () { 
if (/Android|webOS|iPhone|iPad|iPod|BlackBerry|IEMobile|Opera Mini/i.test(navigator.userAgent)
    && window.matchMedia("only screen and (max-width: 760px)").matches) {
	let sigArr = Array.from(document.querySelectorAll('[attrtype=signature]'));
    for (let i = 0; i < sigArr.length; i++) {
		fixArr.push(Array.from(sigArr[i].querySelectorAll('input[type=button]')));
    }
    for (let i = 0; i < fixArr.length; i++) {
      for (let j = 0; j < fixArr[i].length; j++) {
		fixArr[i][j].addEventListener("click", function() { $('#form-signature-mobile-dlg').attr("style", "top:0px; display:block; padding-top:50%").find('.modal-dialog').attr("style", "transform:rotate(90deg); transition-duration:0s; position:fixed; width:34em; height:12em;")});
      }
    }
}
  });
/****************************************************/
/****************************************************/