The bug
There is an inconsistency in how the help information for /fill and /clone are displayed when it comes to selecting the range of blocks.
/clone <begin> <end> ...
/fill <from> <to> ...They should probably be the same (preferably <from> <to>).
Can confirm in 20w49a. I've attached a screenshot which shows this issue. Additionally, if anyone wishes to reproduce this, you can use the following commands and observe the output of each of them.