Why does my PC lose data in sleep mode when I unplug the power strip?

If you're looking to keep your session alive when unplugging, you're right thinking of hibernate. Sleep is cool for short breaks but won't save your stuff through a power cut. Think about using hibernate for those longer absences. It sounds like that's what your old PC did, which could explain why everything was fine then.

It sounds like there's a bit of confusion around sleep and hibernation modes. Sleep mode keeps what you're working on in RAM, which is volatile, so if you cut power, it all disappears. Your old PC might've been using hibernate instead, which saves everything to your hard drive before shutting down. Hibernate can recover your session even after unplugging. Have you checked if your old PC had hibernate enabled?

Yeah, sleep mode isn't designed to work if you unplug everything. RAM needs power to retain its data. Sounds like maybe what you need to enable is hibernate, since it saves to disk and can recover your session even when there's no power. Just curious though—why are you switching off the power strip?