When I post a message on a channel, and people do a "reaction" I get a notification though my Activity Feed to say that "Michael+3 reacted to your post". Thats great. But when I look at the actual post in the channel, there is no reactions on it, until about 15mins later. Even if I log out of Teams and come back in, Reactions on the actual post don't appear. It seems to be a very long delay, considering the notification that someone reacted is immediate. We use the Reactions as a really quick and simple way to do polling, ie. heart = yes. sad face = no. It enables us to see numbers at a glance in these simple yes/no scenarios. But it's not so quick when you have to wait so long for the Reactions to update on the actual post. It didn't used to take this long. Using Desktop App 1.4.00.19572.
If the delay is there too you know it's something going on with the service and not your specific client. If it seems to be affecting only the desktop app you can try and reinstall and/or clear all content in %appdata%\Microsoft\Teams to "reset" it (just copy that and paste via Start).
If no success you should reach out to your IT admin so they can open up a support ticket or troubleshoot further.