Last frame of film dissolve flashing back to full picture of clip

Hi all, any help would be hugely appreciated! I've currently got a multitrack video that I'm editing with 9 seperate sources (in a 3x3 grid) all dissolving out to black before 16 sources (4x4 grid) dissolve in. This is using the film dissolve effect. On previous transitions in this same project, I'm having no issue, but on this particular transition, on the final frame before the 4x4 grid starting to dissolve in, the video flashes to the video without any dissolve (full brightness). I've checked the dissolve and reaches right to the end of the clips.

Any help would be amazing (the full edit it finished except for this issue and it's due tomorrow.