With the heating unit upgrade installed, the oven's GUI does not display how much energy it has. WAILA or TOP does display, however.

For me the upgraded oven does have an energy bar, but it will always show "Energy stored: 0/10000" despite it actually having energy. To verify that the information the interface was giving me was misleading, I had to go turn the energy module of waila on when I previously had it off. I cam here originally thinking that it was just not getting power.

TIL I added an energy bar and forgot about it

Energy showing fine for me on both single- and multiplayer.

I didn't add an energy bar because I didn't see an immediate need for one. The oven is not a "battery" after all. I think since pretty much everyone has either WAILA or TOP installed, that's good enough.

The issue was that wth TOP it didn't display energy levels - didn't pass them to the display mod.

Sorry, I don't understand. What display mod?

That One Probe.