Created attachment 121346 [details]
Zip file containing 3 screenshots about the problem

SUMMARY
When you set icons to individual sorting types, like an icon to sort by name, another to sort by size, you can do this by changing the icon of the "name" toolbar for example, the sort by toolbar icon gets changed when you use that sorting type, if you use sort by size and you have set an icon to the "size" toolbar the sort by toolbar icon gets the same icon, pretty neat, you can know which sort mode you are on.
The problem is when you sort by another thing that doesn't have icon the last icon used keeps, for example you were sorting by size (and it has a different icon) and after it you change the sorting type to sort by Rating, the icon to sort by size doesn't change, you may think you are still sorting by size. Which makes me think it's a bug is when you close Dolphin and open it again it shows the default icon of Sort By if the sorting type doesn't have icon, which could happen immediately when you change to a sorting that doesn't have an icon. Changing between sorting types that has icons works great, the icons are changed immediately.

STEPS TO REPRODUCE
1. Add Sort By toolbar
2. Add some sorting types toolbar to change its icons ("name", "size", "modified")
3. Change the sorting type between the ones you changed the icon (the Sort By toolbar icon will change to it too)
4. Change to a sort that doesn't have an icon, the last icon will stay (instead of going back to default Sort By icon)
5. If you close and reopen Dolphin if the last sorting didn't have icon the default will get used (could have happened without needing to restart the application)

OBSERVED RESULT
When changing the sort type to one that does't have a different icon, the last icon of Sort By toolbar doesn't get back to the default icon.

EXPECTED RESULT
When changing the sort type to one that does't have a different icon the Sort By icon come back to the original one.

SOFTWARE/OS VERSIONS
Operating System: KDE neon 5.16
KDE Plasma Version: 5.16.2
KDE Frameworks Version: 5.59.0
Qt Version: 5.12.3
Kernel Version: 4.18.0-25-generic

ADDITIONAL INFORMATION
Images as attachment to better demonstrate the issue