The documentation's indicates that, "if you want your base font size to be different and not have it affect the grid breakpoints, set $rem-base to $global-font-size and make sure $global-font-size is a pixel value" (http://foundation.zurb.com/sites-6-docs/global-styles.html#font-sizing). I've done that but it actually does affect the grid sizing.
As you can see from the attached screenshots, I have set $global-font-size to 22px; and $rem-base to $global-font-size. In the attached browser screenshot, you can see that the "small" breakpoint is being applied before hitting the 550px point (the point I defined in my breakpoint variables, also attached).
The documentation's indicates that, "if you want your base font size to be different and not have it affect the grid breakpoints, set $rem-base to $global-font-size and make sure $global-font-size is a pixel value" (http://foundation.zurb.com/sites-6-docs/global-styles.html#font-sizing). I've done that but it actually does affect the grid sizing.
As you can see from the attached screenshots, I have set $global-font-size to 22px; and $rem-base to $global-font-size. In the attached browser screenshot, you can see that the "small" breakpoint is being applied before hitting the 550px point (the point I defined in my breakpoint variables, also attached).