I am using LS with parent-fit, object-fit and respimg. Some interesting "issues" arose when cloning an element that contains an image already handled by the plugins:

If cloning and appending said element somewhere where it has different dimensions via CSS, the auto-sizes functionality does not do it's job out of the box. Since LS auto-detects elements added to the DOM, I wondered what exactly in the cloned nodes was stopping that, so I removed the lazyloaded class and readded lazyload. Modern browsers started working as expected, but IE11 din not(It's stuck with the background src it had before cloning).

var source = $(this);
var content = source.children().not('.lazysizes-display-clone').clone();
content.find('img').removeClass('lazyloaded').addClass('lazyload');
stage.find('#team-info-wrapper').empty().append( content );

My next idea was adding lazySizes.autoSizer.checkElems() after appending the element, and, again, modern browsers cool, IE bad.

I then did a quick check without object-fit, and everything was fine in IE, so I guess it's with object-fit that the logic stops.

I'm thinking that both readding the lazyload class and calling lazySizes.autoSizer.checkElems() should, in turn, trigger a reassessment of the bg image implemented by the object-fit.

I hope this all made sense!

Cheers and much kudos for this excellent stuff. It's become an essential part of my toolkit!