Interesting mine is finally working again after months of not working correctly. I had reported a bug on Dec 2nd 2018 with the following information:
iCloud.com completed reminders do not appear when click show/hide option (arrow)
Steps to Reproduce:
Complete some reminders so there are some in the list. Then click the view/hide arrow to display completed ones.
It is expected (and used to be) that completed items will appear in the list and allow to un-complete by unchecking the complete box.
Now it shows total completed number count but when select to view, the list is blank and never shows any items. Also a search never finds completed items anymore either. After a few tries, most of time the completed count goes to zero until you refresh the browser.
I just tried it due to your post and not only is it working but the issue above seems to have been corrected now as well! BTW it always worked from my devices just not iCloud.com. Maybe clearing your browser cache will assist with your issue. I would try that.