My hubby went in for an interview and was told he got the job so he told his other prospective employers that he was no longer interested. Before he could arrange a start date, they ghosted him. He tried to call but it went to an outsourced helpdesk that told him they would create a ticket and he would get a call back. No call after several days. He physically went into the place and the hiring manager seemed flustered that he was there and told him they would contact him. After two weeks from when he was told he would get the job, he finally got a hold of the guy he interviewed with and was told they gave the position to someone else because he was "unreachable".
Problems like this are the reason why I don't hold loyalty to any company unless they've proven their competency. The ones that are good rarely hire because the employees don't want to leave.
That's terrible, I hope it all worked out, but absolutely never say anything until you've both signed a contract unless you're looking for a counter offer, which is risky AF.
People pull out of informal agreements all the time, it's not an employer thing - legal issues, real estate, appointments, competition prizes, dates...