REW trace merge giving unexpected result

Hi, apologies if this has been asked an answered somewhere else but I couldn't find it. I'm trying to merge a diffraction corrected Nearfield response with a gated Free field response using Trace arithmetic in REW. I've done this previously and it worked fine but for a while now I have unexpected behaviour shown below. I'm merging the NF red response as B to the FFblue response as A at 300Hz. The resulting green line is gated the same as the FF response when I'd expect it to match the red response down to 20Hz. If I increase the time window the green curve just drops off quickly.

What's happening here and has something changed over the last few months?

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thanks in advance