Thanks @hube2
We did manage to identify the issue. An editorial request was to make the Excerpt field mandatory, which was causing issues with the acf-field post_type saves via the wp_insert_post_data filter. Once we located that and put an exclusion there for this post_type regular service resumed.
It was definitely a weird one, and I appreciate the advice. Originally I had checked all save_post filters, it was a colleague who found it in the end.
