I was halfway creating an issue for this on gitlab, when I realized that Linux Mint is not officially supported, and before creating the issue, it would be nice If I can get some confirmation from someone with another OS.
I’m using Fedora Linux. I tried your experiment and the “cross” icon disappears when the window becomes so narrow that the text of the message is clipped.
Screen resolution of 1280x1024 works but 1920x1080 or higher is recommended. At lower resolutions toolbar buttons may be hidden but corresponding commands should be reachable via menu.
So I’m not too surprised that making the window so narrow results in loss of icons.
I am not concerned about Icons. The trouble here is that without the cross, the infobar can’t be dismissed. This makes it even worse for people with a low resolution monitor.