@Carole849
Sorry to hear this happened to you. Yes, the IB setting is a problem.
When I started hosting in my own home, I listed one room. The suggested price looked good to me but instant booking did not, so I made sure that box was unchecked and then I went to bed...
I woke up in the morning to find that not only had the system turned IB back on, but that 'smart' pricing had slashed the rate in half and so, of course, I had lots of bookings due to the ridiculously low price. The first one was only for one night, so I honoured that, but there was one for two months also! Luckily, the guests were understanding about it when I contacted them and told them the error.
Some time later, I felt forced to use IB because Airbnb were really starting to push it and burying my listings in the search results. I went from being always fully booked to having no bookings at all, so had no choice.
More recently, I decided I was fed up of IB and would trial turning it off. I did this one by one for each of my three listings and found it didn't affect bookings anymore (I now only host long term guests, so it's less of a problem).
However, after I turned it off, the system turned it back on for one of my rooms, so I know this can happen. Luckily, I spotted it and turned it back off before anyone booked.
I am glad I didn't have to deal with Airbnb CS about it though as that has seriously deteriorated in the past few years, especially since Airbnb laid off lots of well trained CS staff at the start of the pandemic and replaced them and subcontracted to external companies. The majority of CS reps now seem to be untrained with the minimum knowledge of Airbnb policy and sometimes also with very limited language skills.
I am not sure what you can do about the situation now though. No, IB should not be on by default, but at least you know now for the future that it's something to keep an eye on. I keep double checking every now and then to make sure it's still off!