Version 1.17a
Playing on a Server but have the same bug on Singleplayer.
So this problem, I have, is that my Fission Reactor Ports don't accept redstone or better said only for one moment and then they don't power the fission reactor any more. and this moment is mabye a tick.
I'm useing this mods:

best regards

wise720

Unfortunately this is not at all simple to fix with the current system. The overhaul will totally change the way the reactors are formed and this bug will go away ;)

Gonna close this as it's basically unfixable on current versions, but rest assured this won't be an issue in the future ;)