Desired behaviour: either they go to the other network when a roboport deactivates, or they wait until they start going low on energy, and go to the original network if that has appeared in the meantime, or they go to another network if the orignal network has not reappeared.
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UPDATE:Something that looks like it is a bug but isn't:
If you follow them, you will find they charge, but don't go in the other roboport. This is because they have nowhere to put their stuff. Put a storage chest down for them to put their stuff and they will go in the roboport.