AVAudioMixerNode sample rate conversion issue

I'm running a simple audio path that takes the microphone input and routes to the main mixer node, bypassing any rendering in my app just to validate baseline performance. I'm noticing that adding an AVAudioMixerNode to conver the microphone input to 44.1kHz for internal processing is causing audible glitches in the signal path. I'd like to maintain an internal sampling rate of 44.1kHz regardless of external conditions and it seems this should be possible with AVAudioMixerNode. Is this not supported?