It’s likely to be this.
The Monzo feed makes no distinction between authorisations and presented (fully-processed) transactions, but it is likely that the initial authorisation was not presented for full processing in time (within the 30 days, likely due to a system glitch at their end) so it had to be processed again.
This shows on the feed as the initial authorisation, reversed by the refund, and then a new transaction as the details are “put through” again, basically. The initial transaction may actually say “Pending” in small text at the very bottom of the transaction information page in the app.
The £1 authorisations were likely to be just active card checks to make sure you had provided them with valid details, so they are a bit of a different process to the actual charge for the insurance.
It doesn’t really matter as long as the final amount you were charged adds up correctly, many other banks may even have hidden the pending unpresented charge from your statement.
TL;DR it is just a function of the card processing system that you sometimes get payment oddities like this. The complicated behind the scenes stuff usually all works out automatically.