I don't think that Canvancement will help. It only reorders things that are already there. My discussions are already in alphabetical order, even without running the script.
I would start with the normal troubleshooting. Clear the browser cookies and cache, try a different browser, restart the computer, etc.
If none of that helps, then I would start looking at the discussions themselves. Is the Discussions/Announcements Redesign (beta) enabled? The default is off, but the local Canvas administrator may have turned it on under Admin > Settings > Feature Previews. Instructors can check by going to their course Settings > Feature Previews.
Does the problem happen on both the home page (if your home page is set to Modules) and on the modules page itself? A complete list of all possible module items is included when the modules (or homepage if set to modules) is loaded. If there are a lot of items, it can slow down the initial load, but it means that when adding items to the modules, it doesn't have to make a separate network call that could fail. This also means that you can (relatively easily) check to see if the items are getting loaded and not displayed or if they aren't getting loaded at all.
To check and see if all of the discussions are being added, then make sure you're on a page where you can add module items. Then press Ctrl U to view the source of the page. Then use the browser's search Ctrl F to find discussion_topics_select. A few lines after that, you will see a list of the discussions. If all of the discussions are included in that list but not all are showing up, then look for something strange where the discussions stop being displayed. Maybe there is some weird character causing a problem.
Another thing to look at is how many discussions are being loaded. If it turns out to be 10.
Finally, and this is a long shot, make sure that the home page / modules page has been reloaded after the discussions were created. Because the list of discussions is delivered when the page is loaded, it doesn't reflect any additions that happen afterwards.