Someone has both your card and your pin. The bank is quite likely to take the view that you have either allowed someone else to see your pin, or have been careless with it, and written it down or told it to someone.
They may not, but they are entitled to.
It's hard to believe that you have lodged a formal complaingt in the two or so business days since you found that it was missing, and that you have had a formal final response and then also passed the case to the onbudsman. Banks do not tend to work that quickly, this is one reason why people are suspicious of your story.
You have posted several times about your home situation, your child, stepchild, and partner who was bad with money and with whom you are going to have a child next month. When somoene asked if this partner could have taken your card, you said that he died six months ago. This seems strange, too, that a good reason that it was not actually someone close to you just turned up when it was needed.
You posted in Match 2021 that your relationship was over, yet in early 2022 were with someone with whom you'd been for five years and with whom you were planning to have another child.
To be clear, none of that is my business, but people are always going to look back for background in a case like this, and wonder when things seem a bit strange.
If you are to get the money back, which you will likely need for child number three who's arriving into your single-mother life next month then you need to make sure that what you put in your complaint is accurate, and truthful.