Cannot Enable Wipe Tower when either of the support extruders is non-zero

[mkatzpdp]

Description of the bug

I am using an MK4S+MMU3.  All with the latest firmware and the latest sliver version (2.8.1).

Please note for this all 5 colors in the MMU3 are PETG.  

Issue follow these steps:

Load a single project into the slicer
Go to the Print Settings Tab
Select Support material on the left
Enable Generate Support Material
Select Multiple Extruder on the left
Change the Support material/skirt/extruder and/or the Support material/raft interface extruder

Enable the wipe tower so the support extruder/color so that filament changes will be clean.
Notice the Wipe Tower Dialog box

All I wanted to do here was have a different color for the supports than the print.  Why is irrelevant.

From what I can find, it is impossible, in Prusa Slicer 2.8.1, to select the extruder/color you want for the supports. regardless of whether the project is multiple colors or a single color.  If you select yes in the dialog box the Support Material boxes revert back to 0 and if you select no, the Wipe Tower is disabled.

Selecting a different extruder for the supports without the wipe tower will cause the having the main colors in the supports and the support color in the main print.

Have I missed something or is this a bug?

I will be cross posting this to the Prusa Community Slicer Forum as well.

Thank you for reading this.

Project file & How to reproduce

Follow the instructions in the description of the bug.

Checklist of files included above

• Project file
• Screenshot

Version of PrusaSlicer

2.8.1

Operating system

Windows 10

Printer model

MK4S + MMU3