Cannot change selection tolerance setting in Preferences

When I go to Illustrator/Preferences

In the Selection and Anchor Display category, the first item is Tolerance with an editable box with the default value of 8 px.

When I enter, say, 12 in that box and tab out of that box, it reverts to 8 px.

When I click the 'OK' button at the bottom, it quickly reverts to 8 px and closes the preferences modal.

When, after having left preferences, I return to Illustrator/Preferences, the Tolerance box reads as 8 px as before.

I have tried turning off all other features within Selection and Anchor Display category to no avail.

I have tried turning on and off smart guides, etc.

Is there another setting which is interfering/over riding this setting?

Am I special, or have others had this problem?

Thanks