A PageSpeed score drop right after updating a theme or a plugin is rarely caused by ImagePipe itself, since the on-demand engine and its permanent disk cache keep working the same way regardless of what triggered the request. The more common explanation is that something changed upstream of ImagePipe: new markup, new image placements, or a caching layer that has not caught up yet. Working through the possibilities in order usually finds the cause faster than guessing.
Check first whether the new theme introduced image placements that did not exist before, a new hero banner, a different product gallery layout, a new widget area. If these images are not covered by an existing preset (product_card, product_page, category_banner, manufacturer_logo, mobile_thumbnail), they may be served at their original size and format instead of an optimized variant, which alone can explain a sudden score drop on the pages where that placement appears.
Check next whether a third-party caching plugin is serving a full HTML page that was cached before the ImagePipe switch happened. Some caching plugins keep pages for hours or days and will keep serving old
Finally, check whether any new images were inserted outside the ImagePipe Gutenberg block, directly through a classic image widget, a page builder module, or raw HTML, since only images going through the block benefit from the automatic preset replacement in the theme. An image dropped in through another path keeps its original file and format until it is moved into a supported block or placement.