Due to certain issues on Facebook's side, Buy Buttons attached to Image or Video blocks in the flows may not work correctly on iOS devices - you may notice Buy Buttons unresponsive upon tapping them.
Messenger app versions affected by the issue include v258, v259 and v260.
You can track this bug's status in the following thread on Facebook:
https://developers.facebook.com/support/bugs/760785317793497/
More information to follow soon.