Hmmm.... never heard of this either. Flying for ~40 years (in sailplanes) & a CFIG for 8 years.
Maybe the "original issue" was sailplanes that had a noticeable pitch attitude change with change in spoliers/dive brakes (add spoliers, nose pitches up, thus slower/higher AoA?). Rather than point that out, the decision was made to not use them in a turn.

Kind of like training that, "Go fast in the pattern so you don't stall/spin", rather than stall/spin training & recognition.