I was looking at relative finder and there were 12 famous relatives. Not even 5 minutes passed and it was cut down to 4 famous relatives. Where did they go in such a short time?
Somewhere between you and the 8 vanishing relatives the tree was edited, removing a link. Use your tree viewer, selecting Fan Chart, and you may spot where the change occurred. It is likely to be just one change.
either that - or because of a load on the system - the number of generations and people the algorithm looked at could have changed between your first and last run.
sometimes because of heavy use on the system - that can vary from one moment to another - the number of generations back that are checked can vary.
(the utility probably has some sort of time limit - for the records it gathers and analyzes. - and due to that time limit - coupled by different loads on the system at differing times can vary your results)
I would not at all be surprised to see if you try it again the 8 vanishing relatives could re-appear
but as other responders have pointed out - it may have been that a single connection (that was changed) - could have been on the link to all those same 8 people. (especially if they were royalty of the middle ages)
Small changes can have big effects.
In any event, the tree you see is just an entry point to much more work: look carefully at all historical records attached to each ancestor and determine if each child-parent relationship is documented.
In one family I study a certain child-parent relationship was published 70 years ago but not a single historical record has been found to substantiate it. The family is otherwise very well documented, with 20, 30, 40 records per person. Cutting that one link removed many thousands of ancestors from the pedigrees of every descendant of that child.