I've added a button for "sort by" to Dolphin's toolbar, which lets you sort the files by Name, size etc.
By default, this button doesn't have an icon (why?), so I chose an icon for it ("view-sort-ascending" from the default "Actions" icons).
The icon is applied properly and appears in the toolbar, but it's not retained:
- If I click on the "Sort by" button in the toolbar and choose a different sorting order, it is applied to the files, and the icon I chose for "Sort by" is discarded immediately, returning "Sort by" to its original state of not having an icon.
- Even when I don't use the "Sort by" button, if I close Dolphin and then launch it again, the icon for "Sort by" is gone as well, and it's again returned to its original state of not having an icon.

Reproducible: Always

Steps to Reproduce:
1. Add "Sort by" to Dolphin's toolbar.
2. Set a custom icon for "Sort by".
3. Click on "Sort by" and choose a different sorting order, or close Dolphin and launch it again.
Actual Results:  
The icon for "Sort by" is gone, and it's returned to its original state of not having an icon.

Expected Results:  
The icon you set for "Sort by" should be retained.

This bug was already in 4.10.0, and it's still in 4.10.1.
(Can't say anything about versions before 4.10, because I didn't try it there.)