Why does my zsh show these faint underscores since some weeks?

These faint underscores at the beginning of some lines are new since some weeks. Is it a bug or a feature? It doesn't matter what commands I execute they appear quite randomly at the beginning of some lines. They disappear, if I resize the window. I am using JetBrains Mono NL Regular 14 in my zsh for quite some time now, but the underscores started appearing just recently. Besides that, I can't spot any option that should trigger this in my .zshrc. Anyone seens this before?