As it says on the tin, I just installed my new switch, and it seems to work just fine (both switches function as expected - the wiring/setup was identical between the old & new switches), but (thankfully), before I touched the switch after turning the power back on, I ran my AC voltage detector across them, and surprisingly there was voltage detected.
I thought maybe this is normal, but after checking all of my other switches, no voltage was detected in them. So now I'm afraid to touch the new switch!
The condo is an older building (built in '63) and has no grounding wire; I made sure to mount the switch to the metal box first before testing, as I assume it grounds itself to its box. Any experts here that can advise?
Here's a link to a photo of the old wiring pre-installation, and the new switch showing current on the buttons: https://photos.app.goo.gl/weTS8YD2wdm3tqVm7