Mirrored part cannot be flatten

Just tried for fun in Sheet Metal and got an interesting question -

Attached Zipped file includes 2 part files- MM_#091 and MM_#098.

#098 is the mirrored part from #091.

#091 can generate flatten, no problem.

But I cannot flat #098.

Is there any trick in the model?

Best Regards,

Wayne

SolidworksSheet Metal