If you type in chat and the text is long enough it will appear behind the armor bar.

I first saw this when I loaded a world from 20w16a to 20w17a and saw this after I got an achievement. I tested it with player text too and it did the same thing. After seeing this I created a new world in creative, got armor, and went into survival, and it still did the same thing. This is in single player. And it does the same thing even if you have a texture pack.