I'm using 2.9.0 on the live site. Opening and saving worked there.

On my localhost / test site, I updated to 2.10.0 first and no luck until I opened and saved K2 settings...

Also this is really neat - nice feature!

Standard vs. True Toggler - explanation for those who are curious and haven't updated yet:

Choose the standard list toggler (as used in Joomla) where you toggle all rows in a list at the same time to either checked or unchecked. Or use the true list toggler (as used e.g. in Gmail) where you toggle each row's checked state separately. In the latter case, when you have checked for example 2 rows in a list and you click (check) the master list toggler, these 2 rows will be unchecked and all other rows will be checked. Clicking again the master list toggler will revert back the initial 2 rows as checked and all other rows as unchecked.