All in the title, but an image helps:
Items up to "Stash" used to be on the left side. This is the default layout and I hadn't changed it.
I am familiar with the Customize Toolbar option and when I press Done it appears that the Flexible Space becomes a zero-width spacer, which is not flexible at all. You can try and drag new flexible spacers yourself and you get more useless nothing-spacers.
It was fine a couple of days ago. Finder of all apps has the same problem on macOS ! I thought that Apple just dropped Flexible Spaces altogether but if that's the case why even bother suggesting it in Customize Toolbar then.
So to me it is a bug introduced in macOS 11.3.1, but I don't know.
Is anyone else facing this?