When something's wrong
Completed things keep showing
Two settings decide where completed things go, and either one can produce a surprise.
Cause and fix
They stay in place and I want them out of the way. Each list keeps its own choice. Open the list's settings and turn off Keep completed inline, and they collect into their own section. Settings Lists & Things Show completed things inline only sets what new lists start with, so changing it does not touch a list you already have.
They vanished and I thought they were deleted. The opposite: they are collected into a collapsed section, usually at the bottom. Expand it. Nothing was deleted.
They are at the wrong end. Settings Lists & Things Completed things at top moves the section above rather than below.
They keep reappearing at the top. Auto-sort places them in a section whose position you control in Settings Lists & Things Auto-sort section order.
Which setting to use
Inline is better on days, where seeing what you finished among what you did not is the record you want.
Collected is better on long project lists, where finished work is noise.