Yes, it was also my guess that it must have something to do with the fact SVGs don’t have size values in WordPress (even though WP could improve that and read them from the viewport values).
But maybe it’s good to know if the notice is triggered by code from ACF, or if i should file a bug report at the WordPress site.
I just needed to do the same, tried out ractoon’s solution but it didn’t work. I managed then with the ACF method which i found on further investigation: https://www.advancedcustomfields.com/resources/adding-custom-javascript-fields/
Hope this helps somebody else.
