I received a reply from CM, it’s below in multiple screenshots.
TLDR: It’s all very generic, most of these things I’ve already done, but I’ll go through them once more and see if anything fixes this issue.
UPDATE
The latest update from Nothing seems to have broadly addressed the very sporadic updating of data (specifically steps) into Google fit that was taking place overnight.
I’ll keep monitoring over the next few days and update. Hope that’s useful to someone out there 🙂