I strongly suspect in most cases it is the banks fobbing people off - they are unlikely to ever divulge their specific security practices (and certainly not details on specific limits if any). The skeptic in me says that it is in fact in the banks interests to make Apple Pay as inconvenient as possible for end users. Legacy banks don't like Apple Pay because they want full control, and Apple Pay doesn't give them that.
I have the same issues almost monthly with NatWest - you'd have thought their security systems would learn and adapt, but they don't, and you can't even forewarn them about a purchase which may potentially trigger their systems so that they can flag it in advance.
I had similar issues Friday morning with Apple Pay though. My initial transaction was approved (Starling Bank), but then I got a declined transaction notification from Starling about 20 minutes later due to triggering a security alert.
There was clearly an issue with things Apple's end because when going online later to change/retry the payment, they were continually trying to make £0 transactions and then instantly reverse them (thanks to Starlings helpful notifications which showed what was happening). Attempting to use other cards all failed with a mixture of declines, and "unable to process transaction" errors.
It eventually took two separate calls to Apple to get a payment method to actually work (ironically NatWest), and they seemed to know that there had been some problems with Apple Pay related transactions.