When I lay down this blueprint that contains a wireless transmitter from the Wireless Circuit Network mod wired to the circuit interface of a reactor from the Realistic Reactors mod inside of a sandbox from the Blueprint Sandboxes mod the entities are placed down, but the wire connection between them is missing. A copy and paste of the connected entities exhibits the same behavior.

Outside of the sandbox it works fine.

I had reported this in this thread Blueprint Sandboxes thread, where @somethingtohide replied:

This appears to be a bug with Realistic Reactors and there's nothing nice that I can do about it. The problem even occurs with standard, unmodded circuits, and it occurs in a few standard scenarios and mods other than this as well. It seems that it relies on some very specific behaviors that only occur during plain copy/paste with a normal player character.

It looks like this behavior even occurs with an ordinary small wood electric pole when it's wired up to the circuit interface of a reactor. When copying and pasting them inside a sandbox, the wired connection is not preserved on the pasted entities.