Medevac

Why is it that some dictionaries record the inflected forms of the verb medevac as medevacing and medevaced instead of the more correct medevacking and medevacked? What happened to that rule that says that verbs ending in c add a k before suffixes starting with a vowel, like mimic, mimicking, mimicked, mimicker, etc.? I think Merriam Webster does a better job countenancing medevacking and medevacked, but it also lists medevacing and medevaced. Medevacing and medevacking have very similar numbers of ocurrences according to this, but medevaced (unfortunately) routs medevacked here.