Actually this works as intended. When you press TAB again you restart the completion, such that only ea is taken as prefix. However one could consider this as not intuitive, since there is no point in restarting the completion. Then one can rebind TAB in a temporary mct-region-active-map (or in completion-region-mode-map), such that TAB does not restart the completion anew but uses the existing completion table and tries to complete using completion-try-completion.