Just happened to notice this at two forums I admin at, and also noticed the problem here on the main SMF site's RSS feed (https://www.simplemachines.org/community/index.php?action=.xml;type=rss), so I know it's not an issue with any of the mods we use.
The issue happens to be in the RSS feed, any time there's the ' in a post, it's now showing a number code. For example, if you have the word "can't" in your post, the RSS feed shows it as "can'#039t".
This issue is also affecting the ampersand in posts too. "&" in the actual post, but now "&" when seen in the RSS feed.
None of this was happening in 2.0.17, so something got changed that broke this and should be looked into and fixed soon.
My bad, wrong section and it's already been posted about.
https://www.simplemachines.org/community/index.php?topic=576601.0
If you want to delete this thread, you have my blessing to do so.