Should Airbnb be responsible if their emails are routed to spam?

I recently booked a couple Airbnbs in Hawaii with my partner and friends to celebrate vaccination. I paid Airbnb using their option of half upfront, half a few weeks before check-in, because hey, why not.

The first stay went without incident. Upon getting ready to check in to the second place, however, I logged on to the website and discovered that the second booking was cancelled. I was aghast, and tried to get in contact with customer service. After many hours I got someone on the line, and was told that the second payment had not go through, and so they had cancelled the reservation.

I was shocked because I was not notified by this at all and would have been happy to put another card down. After digging, I found that 5 emails from automated@airbnb.com had landed in my spam inbox. I would have never thought to check the spam folder, because in the meanwhile I had received many emails from Airbnb about our other reservations, and gotten like 20 texts with details of the other reservations. Why hadn't they just used the other email or texted?

In the meantime, they had relisted the unit and it was immediately booked by someone else. They also took the $3000 we put down on the unit because it was cancelled after the date of a refund.

This had severe consequences for us. We couldn't find a comparable place to rebook and so our friends cancelled their whole trip (they were just flying in for the latter half). We are now spending our remaining time just trying to cut our losses and figure out how to end the vacation early, which has made everything extremely stressful. Overall I think we'll lose over $5k. I was also going to propose on this trip, but will not anymore because we're both so sad. Airbnb's customer service is completely unresponsive and unempathetic.

Anyway, to the question. Who's at fault here? Am I dumb and deserve this for not checking my spam folder all the time? Is it GMail's fault? Is Airbnb at fault?