Interesting just some quick observations
With no solder on the board in the intended spots for the tabs, and the tab themselves being relatively straight it appears that the solder iron never came around to them, there's also something on one of the tabs (possibly both)which maybe something used to anchor them to the battery before they were soldered.
The mark on the battery is not really corrosion but dirt mixed in with that material that is on the tab.
So it does appear to be a factory error