It sounds more like a hip flexor strength issue to me.
Other than the typical knees-past-elbows, straight leg raises, L-sit progressions, etc. from gymnastics, make sure that on any weighted exercise where you stand up that you fully extend your hips to straight (maybe even slightly past straight) with a strong glute contraction to at least stretch the hip flexors to "normal" in the standing position.
Add PNF activity (contract-relax) to any hip flexor stretch to strengthen in the elongated position.
More vertical jumping, tuck jumps, and broad jumps.
Do all your pullups as "L" pull-ups. That used to be the only way I could do them in my garage, I believe it really helped me in certain ways.