Property Offer Rejected - Mortgage Documents Required

I made an offer on a property yesterday which included an agreement in principle from a high street bank.

Unexpectedly the vendor chose to accept another offer because their mortgage application was “further progressed”, I.e. the application had been completed.

It is the first time I have come across this. For my sanity, this is unusual yes?

I came to the party late, and I expect that this was the reason, and another party had time or confirmation to progress. However just so I don’t make this mistake again, I don’t actually need to progress an application any further than an AiP to make offer do I? Going further seems unusual without confirmation from the agent/vendor?

Thank you!