• Version: v6.0.0-rc.3
• Platform:
• Subsystem: repl

It seems that tab completion only works if the element to be completed is at the beginning of the line.
Examples:

• > buf<TAB> completes to buffer
• > <SPACE>buf<TAB> does not complete
• > buffer.<TAB> completes to the list of buffer properties
• > var buf = buffer.Slow<TAB> does not complete