Well in my experience when the player model defaults to the T-pose, which is the pose the the bind pose was added to, then that indicates that there is no valid animation key frames at that position with in the animation stream. Your fix is valid as a fix as you are removing the null frames. To confirm I’ve run across the issue from time to time and one cause is when imported more frames than animation is imported so if no offset animation keys are available then the default T/A pose,or the bind pose, will trigger.
Not an uncommon issue that Epic purchased packages would contain these issues if on the authoring side the number of frames are exported above the total number of transformation frames.