Other than a short in the field circuit, there is ONE other remote possibility. Since some of these aftermarket units may well come from (cough, gag) questionable sources, it just might be that the METALLIC ALLOY of the rotor core is not what it should be. It MIGHT just be that some units RETAIN MAGNETISM far better than they should, and that when reversing the field, the residual magnetism in the core (at least for a short time) will be "fighting" the polarity of the field due to the swapped field connections.

Back in the days when rebuilders were "cheapy" modifying older frames for isolated field, I used to see a fair number that were mistakenly grounded.