Multichannel, Multipart exr rendered from Mantra not listing any layers in the Layer Selection dropdown in Nuke

[lookbothways]

Hi, Regular mutlichannel exrs are fine - and I can pick colours in the Cryptomatte node in Nuke on multipart exrs when I shuffle the channels, but the dropdown is empty and there is no crypto listed in the metadata key. Is this a known incompatibility?
Does cryptomatte not support mulitpart at all?