Q: What does a quick change from 'Under consideration' to 'Decision made' back to 'Under consideration' mean for Nature?

Such quick changes may mean a glitch, but given that it’s a Nature journal, that’s unlikely. :-)

So, it probably means that the associate editor (AE), who handles the manuscript through the review and publication process, had a look at your manuscript and made a decision on it, most probably for rejection. However, when they communicated their decision to the Editor-in-Chief (EiC), who makes the final decision, it was decided to reconsider your manuscript. This is known as a rescinding.

The EiC may have seen merits in your paper after all (or a fit, if that was the issue). Alternatively, they may be considering that you submit to another journal within Nature, where there may be a better match.

Hopefully, the exact situation will become clear with the next status update. Until then, you may find it worthwhile to go through the following resources about journal statuses:

• Can we write a courteous mail asking about the status change from 'Decision Made' to 'Under Consideration'?
• What does it mean if the status is 'under consideration' for two weeks?
• Keep calm and wait: A guide to understanding journal statuses [Handbook]

All the best for whatever’s next! :-)