Due to certain issues on Facebook's side, referral links may not work correctly on iOS devices currently, which mainly affects Messenger Ref URL Growth Tool in ManyChat. QR Code Growth Tool, SMS and Email messages with referral links in them can also be affected.
You may notice wrong messages being sent (Welcome Message for example) instead of the Growth Tool's Opt-In Message, or no messages being triggered at all after clicking referral links.
You can track this issue's status in the following bug thread on Facebook: https://developers.facebook.com/support/bugs/1097014660640216
More information to follow soon.