VST problem whith ambisonic layout

I have a problem when the VST plugin try set an ambisonic output according to the number of channel. setPlayConfigDetails is called, then there is a test if the number of output wanted is different from getTotalNumOutputChannels. It’s always the case with reaper when I try to initialize the plugin at the ambisonic order 4. The function try to initialize the bus with setChannelLayoutOfBus. But the function AudioChannelSet::canonicalChannelSet return a layout not supported by my plugin … I only accept ambisonic layout which is not support by this function (it considers only discrete layout).
Is there something to do when the number of channel change? Is there any restriction whith VST plugins (it works fine with AAX)?