I learned from Canvas Support today that this is a known issue if you have "too many" assignments in your course. I also learned that fixing this is not on the development roadmap, so we will have to live with it until and unless enough of us scream about it.
In the meantime, here's what I have found:
What *sometimes* works is to re-open the assignment, make a small change, and then save it. Then look for the assignment using the Module "+" pulldown.
That worked for me in one case several weeks ago, but not today.
If it doesn't work, Instructure has no solution except to suggest you do a workaround. What you'll need to do is create a new assignment *from* the Modules page (so it shows up in the Module). Then enter the text of the assignment. If you've already created an assignment that you can't add from the Modules page, copy/paste the text from that assignment into the new one you created from the Module page.
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