Illustrator often fails to build a smooth blend for paths with mismatched number of anchors
Take a look at the file attached and watch the images attached.
Two curves are blended: one has 4 anchors, another one has 5 anchors.
The blend Ai build has weird irregular slits in the middle.
If you expand the blend and select problematic anchors, you’ll see that positions are correct, but some handles get interpolated wrongly.
This happens often in many other scenarios.
The current workaround is to add more points to blend, trying to guess and control which point get paired with another.
Another workaround is to cut curves in pieces and blend in chunks.