The @ in Discussions doesn't work correctly

Issue Summary

When adding someone's name in the Discussion field the @ only works when it's the first thing you type.

Eg, @greg, can you take a look at this will work
can you take a look at this @greg will not work

it shows that @'ing 2 people will work at the start of the text but the 3rd person doesn't work. Though this person exists.

Steps to Reproduce

go to feature > Discussion
Test 1) If I put the user - @user1 @user2 @user3 @user4 <Message> - Result: all users are searchable.
Test 2) If I put the user - @user1 @user2 <Message> @user3 - Result: user3 is not searchable

Expected Results

all users are searchable, even the user is added after the message

Actual Results

unable to searchable, if the user is added after the message.

Workaround

Add all users in the front of the message.