The amount of stuff you get from deconstructing something (as well as when bashing something) is randomly chosen (with certain bounds), though I am unclear on the actual mechanics, and with the way it’s set it seems like you should always get at least some sheet metal either way.
edit: okay, that has to be the single most ridiculous outdated implementation of a feature I’ve seen here so far. It’s literally written in code, per specific items, what you get from disassembling. And yes, there is no sheet metal for the oven, so that’s most definitely a bug. I guess it’s a leftover from when these things were still defined in code rather than .JSONs.
edit2: PR’d a fix for this: https://github.com/CleverRaven/Cataclysm-DDA/pull/6845 Just adding the sheet metal in, but am really considering what can be done for a more modular system.