BigSur mail toolbar customization not saving some changes

When trying to customize the toolbar in the Mac Mail app under BigSur beta (all betas thru bate 6) some items can be moved, others can not, and still others don't appear to show at all, such as flexible spacing.

I assume this is just a development bug, but am wondering if anyone else has experienced this issue.

Also, there is no "tag" for Mac Mail in these new forums.