Seems a bit harsh if there is as you say CCTV evidence you did your checks.
A few points, was it the nut itself that failed, the threads, or the stud?
I have in my life (very rarely) seen a nut break (crack or split), this only ever happens due to a manufacturing defect.
gov.uk/government/uploads/s … -lives.pdf
Makes interesting reading, particularly the last bullet point under care of wheels on page 3
How are your checks conducted? Are the firms checking and processes for remedial action adequate?
If not you may have a case for unfair dismissal should it come to that, I’ll leave you to read and ponder if you should be taking up any shortcomings with your firm / boss.