I was checking out the Oversampling class on a single channel. The type filterHalfBandFIREquiripple works great.
However, when I use the filterHalfBandPolyphaseIIR type, the engine buffer has a size of 2.
In line 285 in juce_Oversampling.cpp: OversamplingEngine (2, numChannels)
the arguments are mixed up. First one should be channels, second one the factor 2.