I recently edited some fields, and noticed my custom permissions weren't working right. So, I went to edit the field. It turns out that I can still select the "custom permissions" radio button, but all of the checkbox selectors have disappeared. Therefore, after saving the field, the custom permissions were deleted. I can still access the settings through the config page for the module, which acts as a workaround, but the regular settings that should be in the edit field page are no longer there.