Leaflet image overlays and iOS 12: partial rendering

Question

My PWA (using Leaflet 1.5.1) is currently running (since 2 years) on Android and iOS without problems, until iOS 12 was released. From that point on I have very weird issues on iPhone and iPad.

Main problem is that image overlays are not fully rendering on iOS devices.

I am setting up an array of overlays:

ovl = loadOverlay(url);
overlay.push(ovl);
where loadOverlay() is:

function loadOverlay(imgurl) {
    bounds = new L.LatLngBounds(
        new L.LatLng(30, -10),
        new L.LatLng(50, 36)
    );

    var ovl = new L.ImageOverlay(imgurl,
        bounds, {
            pane: 'imagebg',
            attribution: "my attribution"
        });
    return ovl;
}

Then, each overlay is displayed this way:

startMap() {
 mymap = new L.Map('CopMap', {
 maxZoom: 6.50,
 minZoom: 4.75,
 zoomSnap: 0.25,
 zoomDelta: 0.25,
 zoomControl: false,
 maxBounds: [
        //south west
        [32.5, -4.],
        //north east
        [46.8, 31]
     ], 
        crs: L.CRS.Simple
 });
    mymap.setView([41.5, 13], 5.25);
    mymap.attributionControl.setPrefix('');

    mymap.createPane('imagebg');
    mymap.getPane('imagebg').style.zIndex = 1;

    mymap.addLayer(overlay[iDX]);
}

This is working fine on Android, while in iOS the image is not fully rendered (not always, but very often). I have checked that the image file is correctly downloaded in the cache, but not fully displayed