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. Settings Lists & <i>Things</i> Show completed <i>things</i> inline is on. Turn it off and they collect into their own section.
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 & <i>Things</i> Completed to-dos at top moves the section above rather than below.
Select all did not include them. A collapsed completed section is excluded from Select all. Expand it first. See select many things at once.
They keep reappearing at the top. Auto-sort places them in a section whose position you control in Settings Lists & <i>Things</i> 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.