Environment Variables fields in "Advanced" section of build step are erased after changing step type from Parallel to Sequential. It can be seen in Audit Log, but shouldn't happen at all.

I checked if that happens from other fields and the answer is: no. But I didn't check transitions between other types of steps, I think it's worth checking, because administrator can overlook that some values are erased upon changing step type which can cause build failure as a result.