Yes, it holds inside each bucket, and it's wider there than pooled. Default profile, heldout_v3, family-clustered CIs:
- choice 3–5 (n=1,002): answered .684 (771) vs declined .303 (231), gap +38.1 [25.9, 49.3]
- choice 6–10 (n=393): .640 (308) vs .200 (85), gap +44.0 [29.7, 57.9]
- yes/no (n=1,032): .740 (438) vs .561 (594), gap +17.9 [10.0, 25.0]
- score (n=548): .609 (445) vs .330 (103), gap +27.9 [18.9, 37.7]
You're right that the mix moves it. Weighted by declines per bucket, the within-bucket gap is +25.7 [19.2, 31.3] vs 22.4 pooled. The pooled number is pulled down because yes/no questions are 59% of the declines, and a declined yes/no answer is still right 56% of the time (chance is 50%). Declined answers sit near chance in every bucket: .30 vs ~.23 for 3–5 options, .20 vs ~.15 for 6–10.
One note on the never-abstain buckets: heldout_v3 has no 2-option choice questions and only 18 with 11+ options, so they barely touch this split. And your 0.51 band is right (I get 236/463); by bucket it's .46 for choice 3–5, .42 for choice 6–10, .65 for yes/no.