It depends how you tell the story

Sorry to reply late.

It is all about how you tell the story. Even it is ONE true story, it could be told in many different ways. First, you have to pretend you just found your issue. You disclose your finding with NO delay. This is called 'team work'. Do not talk too much about details. Only point you need to make is that you realized the issue and have been working hard to figure it out. It is nothing wrong you suspect the other one gave you wrong format at the first place. Again, before the root cause if found/verified, you can speculate anything and share your speculation with the team. You did nothing wrong, even it is now found you screw up something :-) Again, the story you want to tell is how hard you have been working on it. Now, the issue has been narrowed, maybe some open questions still remain, you are working with the team ... no need to apologize, but may say sorry for confustion, etc ...

Just my 2 cents, hope it makes sense and helps