I tried this in the current v1.14 beta, using a "fresh launch" DF-26. The missile and the RV both flew as expected, on a parabolic trajectory. For instance, at a distance of 22nm from the impact point, the RV was still at an altitude well over 9km (and falling rapidly, of course).
However, I do recall that this issue was present at a much earlier version, and it was fixed.
Any chance that the actual launch and trajectory was run in a much earlier build, and thus you only observed it in B972.14?
In any case, on the current build this problem does not seem to manifest itself. Can you please confirm?