Ketone/aldehyde synthesis from N-acylazetidines or aziridines
I think your suggestion about the stability of the tetrahedral intermediate is correct, but it's not that the tetrahedral intermediate itself has stability. I think it's because the collapse of the tetrahedral intermediate leads to a less stable intermediate.

My key insight for rationalizing this is that lithium hydride reduction of amides generally affords amines. We generally prefer to eliminate the lithium-oxygen adduct from the tetrahedral intermediate, forming an iminium ion.



With the two substrates you've provided, the iminium ion would be very highly strained because of the geometry of the small ring. I think collapse of the intermediate is thus disfavored. On work up, we actually kick out the amine to collapse the intermediate, providing an aldehyde or ketone.