Why is my incremental crawl not stopping on SharePoint Server 2016 on premise?

Question

I've had a SharePoint Server 2016 search crawl set up for about a 18months now. We have recently had issues with the search incremental crawl. I've restarted servers, stopped the crawl, restarted it. When I restart the search it kicks off a full crawl, this completes, then the incremental crawl starts at a scheduled time and should take around 3 minutes but it is on 40 hours now and continues.
There seems to be quite a few security updates being triggered? Before the issues it was a consistent 417 but now it is over 1 million...

The search application is split between four servers and all components are working with green ticks.

Any help is really appreciated.

Thanks.