Electrical – What could cause a digital circuit breaker finder to not find a breaker

You have to use the right size wire nuts, and you have to put them on correctly. If you recall correctly, you have the right size, so you may not have put it on firmly enough. Another common problem is not having all the wires at the same length, so that one is not as well connected. Only when connecting stranded and solid should the stranded be just a bit longer/further into the nut than the solid wire(s).

You may find it easier to use a nut with "Wings" that make it easier to twist forcefully.

Do NOT pre-twist the wires. The wires, stripped to proper length (per instructions of the wire connector - look on the box) and aligned with each other are twisted BY the wirenut as it's installed (some instructions may say "pretwist optional" but in point of fact I find that it works best if the twisting is all done by the wirenut.)

Tape does nothing to prevent or solve a poor connection.

You don't need to decrypt breaker indicators. The fact that two breakers trip is all you need to know.

Why would one thing trip 2 breakers? These two circuits have their neutrals crossed, tied together, or interacting in some way. Perhaps one device which takes its hot from circuit 1 is taking its neutral from circuit 2.

Hots and neutrals are supposed to be monogamous -- they're only supposed to partner with each other, ever. That's always been in the code. But until recently, crossing neutrals "worked" - it didn't cause any detectable problems.

What happened "recently" was adoption of GFCI and AFCI. Those absolutely require hots and neutrals be monogamous. If current comes back on the neutral in an unexpected way, that makes it trip.

So you have to go through each circuit, look closely at every box, and find the place where a neutral doesn't match its hot. It's a tedious job, and I've done it. If it helps, get gray tape and put it on the neutral from one of the circuits. Gray is an allowable color for marking neutrals.