So when I put my reactor online, I did notice the Steam Engines stopping. Which is as expected.

But then I deployed a big, electricity-guzzling production line for Steel. And after it runs, I noticed that the miners are now complaining of "Low Power"

Panning to the Steam Engines, I noticed none of them are starting. Though they are fueled (with coal) and primed (with water+steam).

Is this by design?

Can't say anything about that without a save file that demonstrates this issue, feel free to send one with low power entities.
Make sure the steam engines are on the same electric network as the miners complaining about low power, and generally that they are in fact connected.

Generally, the only thing this mod does is fiddle with priorities, it should never make power generators not work when they're needed.