This may be getting into EU retconning, but who's to say Vader doesn't have multiple spare helmets? Maybe different ones are designed differently internally and that particular day in TESB he just happened to be using a "one piece" helmet, or a "two piece" version that one day in ROTJ. Maybe he started with the two-piece in ROTS, changed to the one-piece somewhere along the way and then went back to the two-piece somewhere after-ESB/before ROTJ.

Of course, the real reason is that ESB is the first time we see him without his helmet so they hadn't fully thought through how his helmet would break apart - they just did what worked for the scene - then re-designed the concept when they got to ROTJ.