This sort of stuff is why I dislike Monzo’s approach of not showing the actual balance in the app, and not distinguishing pending transactions more clearly.
But, @anon23935806 is correct, and it’s apparently part of the overdraft agreement that pending transactions aren’t taken into account when calculating overdraft charges. Source
Also, it is my understanding that your pre-auth will automatically be released after 7 or 30 days (depending on the type of pre auth they did - see this and following posts). This will appear like a refund to you, though it technically isn’t.
For that sort of purpose (long term pre-auth, especially when largish amounts of money are concerned) I really prefer my credit card …