Answer
There are two separate steps needed, not just one cache refresh. First, RMB on the custom field itself and Synchronize it — after any definition change (like editing its SQL), the custom field shows as unsynchronized until this is done explicitly. Second, on the actual page/window displaying the field, RMB > Custom Objects > Reload Configuration — window layouts are cached client-side for performance, so this explicit reload is what makes the synchronized change visible on that specific screen. Doing only one of the two (synchronizing without reloading the page's configuration, or vice versa) typically isn't enough on its own.