Until around the time iOS 12 was released, every time I reacted to a friend or received a reaction from a friend to an Activity achievement on the Watch, I was seeing these cards in iOS Messages within the chat thread with that person:
At some point they started to only display as empty bubbles, regardless of whether I’ve sent or received an Activity share achievement. I can see the body of the message, if it’s the latest message in the thread, in the list view of threads in Messages, but when I click the thread I now see empty bubbles (specifically only where the activity sharing cards should be):
The empty bubble only shows up on my phone, it looks how it is supposed to when I go to messages on my Apple Watch though. Same with iPad and Mac.
(I borrowed this text from a post on Apple's Support site since it is the exact issue I have but there was no solution)

At some point they started to only display as empty bubbles, regardless of whether I’ve sent or received an Activity share achievement. I can see the body of the message, if it’s the latest message in the thread, in the list view of threads in Messages, but when I click the thread I now see empty bubbles (specifically only where the activity sharing cards should be):

The empty bubble only shows up on my phone, it looks how it is supposed to when I go to messages on my Apple Watch though. Same with iPad and Mac.
(I borrowed this text from a post on Apple's Support site since it is the exact issue I have but there was no solution)