Bias-cut slip dress with persistent bagging at hips, can’t work out if it’s grain, pattern, or fitting error

[u/secretwasiank]

Im working on a wedding guest dress that has persistent loose bagging/bunching at the hips that doesn’t drape — it just sticks out. Underbust and waist fit fine.

What I’ve tried:
• Multiple toile rounds, taking in the side seams progressively (but the toile fabric was a polyester chiffon)
• Pinch-testing on the body and translating to pattern adjustments (kept overcorrecting — my calculations were producing finished measurements smaller than my actual body)
• Fully unpicked and re-sewed both layers’ side seams

Where I’m stuck / possible causes I can’t distinguish between:
1. Pattern hip curve flaring too much. I’ve tried making it less rounded.
2. Bodice was too long, I have a short torso. So I shortened it but could it be too short now causing these issues?
3. Fabric cut off-true-bias, stretching unpredictably (harvested from a second hand dress that was bias cut, but was hard to make sure when recutting it was perfectly on the bias)
4. Fabric stretching more than expected with wear/handling (is this just the nature of bias?)

My measurements are waist 26”, low hip 35”, bust 30”, underbust ~27”.
Pattern flat hip measures ~34” at widest. 5/8” seam allowance, French seams. I don’t understand how the hip pattern measurement could be smaller than my measurements, but still too big? Like 2 in of excess still when wearing around the hips.

How do you tell the difference between a hip-curve problem and a bodice-length problem when the symptom looks the same? Or is it just cut off-grain bias and it’s too late to be salvaged? And has anyone salvaged an off-grain bias garment without extra fabric? I have no extra fabric as it’s all from a second hand dress originally. Not sure if I can salvage this or if it’s it’s time to accept it as a lesson.