JUCE always uses the same channel order internally no matter if the native AU/VST/AAX layout has a different order. JUCE’s plug-in backend will always re-order the channels for you. The “JUCE channel order” is always in the order in which the channels appear in the ChannelType enum list here.
So for 5.1 surround sound, the channel order of the buffer in your processBlock callback is always: L R C Lfe Ls Rs - regardless of DAW or plug-in type.
If you are getting an incorrect layout in some DAW then this is likely to be a JUCE bug (or DAW bug). I’ve just tested 5.1 Surround in Logic Pro and the channel ordering seems to be working fine.