Deforum sequences suddenly 'flipping' between consecutive frames?

[optisynapsis]

I doubt this is an issue with deforum itself, but may have something to do with a specific setting in automatic1111 that I've changed :

Most of my animations with deforum (mainly interpolation based) have had smooth transitions between frames, but now it appears that regardless of settings in deforum, images in a sequence appear to repeatedly flip backwards every 2 frames, still progressing, but the order seems to be odd.

Any idea what setting might have led to this behaviour?