I've noticed this, too. The original frames are displayed using the adjustments made in ACR, while the final stack is not. When saved, the ACR adjustments are restored and the image appears in Bridge as expected.
I believe this only happens when stacking RAW images, and can be avoided by saving RAW images as TIFF images first. This can be done in Photoshop or from ACR in Bridge.
I would add that re-applying the ACR adjustments to the completed render for would be critical for retouching.