There seems to be some differing times depending on the transaction as a general rule the transaction drops off on the 8th day but can be 30 days for some types of payment.
Monzo have already told me that TfL payments take an extended time to drop off because they work differently to normal contactless payments and this can be up to 12 weeks!
I have had to dispute this pending transaction now to try and get it removed from my account.
Fortunately for me it’s only £5.25 but I do wonder how many other people have been affected with larger amounts and haven’t noticed!