If the window title provided by hyprland contains invalid UTF-8 then hyprland/window module fails to update completely and the original text from the previous window (that is no longer in focus) is preserved.
Preferred behavior: Maybe render what you can and indicate the problematic areas with replacement character (U+FFFD)?
Even skipping the {title} part in the format string would be OK but not indicating the focus change at all is problematic.
(noticed on Waybar v0.13.0)