That's a known bug that's been around for a while now.

The workarounds for it include testing for an error when performing your
operation and if it's that error checking for the complete flag and clearing
it, moving/copying the item and then setting the flag to complete on the
copied/moved item. It's a hack but it lets things work. You can also test
for flag complete before the operation but that's extra work for the
majority of items that won't be flagged complete.