Samba seems to have added an unwanted %U (that I have never typed or selected) into a the path suffix on a newly created share.
I'm aware that suffix can't be changed if any users are connected to the share, so changing it should be easy.
But even with samba disabled, and the share disabled, when I re-enter the share GUI, it's greyed out and won't let me remove it. See attached screenshot
(A second, pre-existing, share with identical params and blank path suffix works perfectly, for comparison)