I played a bit with both plain Moose variant and MooseX::Declare one. I used Devel::Trace output to see what happens behind scenes.

It looks like initialize_instance_slot for attributes is not called when MXD is used -- the MooseX::UndefTolerant::Attribute is using it to plug its behavior. I have no idea why it is, though. Might be just bug on MXD part.

I've seen quite a number of strange behavior (for instance with use overload) with MXD that I switched to plain Moose some time ago. Not only it get large speedup, there are also less surprises.

-- regards, Roman