Steps to Reproduce

Run keikai 6.0.0 FL
Open new sheet
paste the following in cell A1

=Sum(1,2)

Open the inline cell editor by clicking on the cell once then pressing F2
use keyboard.
Use left key to navigate the cursor here:
=Sum(1,[here]2)
(alternative: use left+shit to select the character "2" in the formula)

Note: same effect using formula bar

Current Result

first formula argument is highlighted

Expected Result

2nd formula argument is highlighted

Debug Information

It appear that the index is offset by -1.
cursor located at the "1" character position highlights the Sum formula instead.
Also affect nested formulas, ex:

=SUM(1,sum(1,2)

Workaround

N/A