at the point of the observed drop I hadn't changed any page URLs, no. Apologies for lack of clarity. I had changed image URLs when they migrated from being PNG

at the point of the observed drop I hadn't changed any page URLs, no. Apologies for lack of clarity. I had changed image URLs when they migrated from being PNGs to WebPs. Following that there was the SEO drop off (within 6 weeks or so)