It’s likely that mismatch is the culprit.
In your new sequence…if you double click a clip to load it into the source viewer. Where do the In and out markers fall?
is your material still marked they way you intended?
Also compare the Clip Duration in the new sequence versus the old one.
I think you’ll find that it is considerably longer in the new sequence than in it’s origin.
Do they have a resolution preset with a frame rate that matches your old seqeunce?
if so that may be the shortest path to getting you where you need to go.
Alex