The bug

The syntax help for the /clone command is missing the case where filtered is chosen as maskMode. In this case the cloneMode and an additional block argument are required.

How to reproduce

Use the following command

/clone ~ ~ ~ ~ ~ ~ ~ ~1 ~ filtered

→ It will show you the syntax help despite the fact that the used command matches the printed required syntax

Suggested syntax help

/clone <x1> <y1> <z1> <x2> <y2> <z2> <x> <y> <z> [replace|masked [<cloneMode>]]|[filtered <cloneMode> <block> [<dataValue>|<state>]]

or

/clone <x1> <y1> <z1> <x2> <y2> <z2> <x> <y> <z> [<maskMode>] [<cloneMode>] OR
/clone <x1> <y1> <z1> <x2> <y2> <z2> <x> <y> <z> [filtered <cloneMode> <block> [<dataValue>|<state>]]