I just updated my theme and the featured images are not showing on mobile when it loads at first, I noticed with another phone that it happens the same thing, both phones I tried were iPhones. It's weird because if I click and go to one page portfolio page and use the back button, then all the images appear. The problem also gets fixed if I turn my phone to horizontal view, and if I turn it back to vertical they stay visible and correctly loaded. I tried regenerating thumbnails with the plugin Regenerate Thumbnails to check if that was the issue but it didn't seem to do anything.
Also, does it look to you that it loads slower than it should be?
Thank you so much for your help!!
Have a great one!