The frame in the Sequence Monitor and in the Source Monitor are not matching

We just locked picture on our short film (hooray!) but are now experiencing something funky.

We're going through the sequence to track where visual effects have been applied. But when we double click on the clip in the sequence, an image from an earlier or later part of the clip appears in the Source Monitor. It isn't on every clip, and it isn't consistent. But it is happening pretty frequently.

The still image in the Sequence Monitor (effectively where the cursor is) should be identical in the Source Monitor. Right? But now this happening.

Has the project been corrupted in some way? Has this happened to anyone? It's strange because I feel like I can't adequately explain it. Simple example below...

The project is heading off to the colorist so we need to be able to send it to them with confidence. Any insight is welcome.

best

Christian