Guys, I'd recommend you to re-submit the https sitemap as I received email that it is fixed and indeed mine is fixed now. If resubmit didn't work for you, then I think you may want to contact Bing. Hope this helps
Still not working for my site on Shopify, I have spent many hours emailing both Shopify and Bing to no avail. I am pleased your site is now accepting https on Bing, hopefully others on the same server are too. It might be IP or location specific.