When inserting an Ambisonics plugin on an Ambisonics track, the channel routing is off. Observed on the Mac version of Cubase.
The first 4 channels (1st order) are somehow identified as channels 19-22, the remaining 12 channels (2nd and 3rd order) are identified as channels 32-43. The behavior can be observed using the 7th order Ambisonics meter by SSA Plugins (see attached screenshot).
The behavior is consistent across 1st, 2nd and 3rd Ambisonics tracks and seems to be independent from the type of Ambisonics plugins used (although I cannot confirm this 100% as the axmeter Plugin by SSA Plugins is the only one that dynamically adapts the visualization to the channel numbers reported).
I have also reported this issue to SSA Plugins but after playing around with Ambisonics plugins from multiple vendors I am fairly certain that this is a bug in Cubase. (Either that or I am doing something utterly stupid.)
While Cubase has most of the functionality for processing Ambisonics tracks natively included (brilliant, btw), this channel routing problem currently prevents the use of Ambisonics reverbs, manipulators or 3rd party Ambisonics decoders such as Waves NX.