Are you 3D printing a new top plate for the psu? Remember that the steel housing also functions to contain the emi generated from the psu components. You could end up increasing electrical noise for other components if your new top plate isn’t conductive and grounded.

I don’t know the answer, but I found this discussion which seems relevant.

https://electronics.stackexchange.com/questions/69666/atx-power-supply-front-end-how-does-this-work

The explanation given says it has to do with the functionality allowing the psu to be used with either 110VAC or 220VAC mains.

Does the psu you measured without input capacitance have a physical switch for selecting 110V or 220V?