After playing around with some heart particle effects I noticed that ice blocks overly in front of them, even though the heart particles are the most forefront object. I traced this back to the source and spawned a wolf 10~ blocks in front of a wall of ice. Then after taming it with a bone it emitted heart particles, which as expected were covered by the ice blocks - even though they were behind it. In the image attached I added strips of solid stone blocks, just to show how the heart particles are meant to overlay.