For greater clarity, it's an issue with Sync for Lemmy continuing to use an API parameter that was formerly deprecated and now entirely removed from Lemmy.
Sync for Lemmy is identifying a post that needs to be marked as read using a single value parameter, but it should use a slightly different parameter that could accept multiple posts.
Seems like it should be an easy fix for Sync for Lemmy to use post_ids instead of post_id.
In the meantime, I guess I'll poke around for an alternate Android client.