In some rare cases iPhone users may receive a text/SMS message containing the link to your card, but they won't be able to 'open' the URL. This is due to a security setting on the recipient's device which does not allow the iPhone to open any URLs from new people.
Two ways to overcome this:
Wait a few minutes - their iPhone might hyperlink it automatically once it completes a few security checks
Recipient to 'long-press' on the message and tap the 'thumb up' icon that appears - that will tell their iPhone that they trust the sender and the URL will get hyperlinked right away (this will also fix it for all future incoming messages)
Important to note, is that the issue is not a Haystack specific one. For these recipients, their security settings do the same for any URL sent via text message from a new person (even if the URL was CNN.com or NASA.gov).
While this is a rare issue, some senders may encounter this issue more frequently - if you do, consider sending your card via QR codes, email, WhatsApp or Facebook Messenger instead - the issue only shows up on iPhone iMessage incoming messages.