I've got a smaller account(33k) and have noticed similar issues lately. It makes no sense as to why visibility fluctuates when my behavior hasn't changed. I have a feeling it's Instagram trying to push us to use their ad system.
For one project 700 likes was the end. I collected those in a few months due to natural growth and then it suddenly stopped. With 700 likes to the page there were only around 150-200 impressions per post (no matter time or day or post content) and Facebook _heavily_ offered me to use their ads to push my views.
Tell me what you want, this is about monetization of their platforms, nothing else.