Hey check your contacts on Google contacts on the web. Look to make sure the missing contacts are listed there. If they are open each one and delete any URL's associated with them like Facebook contact syncs etc. That should restore them to your phone. I had a big issue with the same thing and that fixed them all. Let us know.
OK, checked google contacts via the web address and all the missing contacts from the phone are also missing there as well. I reinstalled them "again", about ten.
I'm thinking they go missing from the phone first then the next sync from google and they are deleted from there as well. I put a test contact on the google web site and it immediately synced to the pixel and also vise versa so that is working as it should.
I rebooted both my pixel and tablet I was using and so far they remain. If it continues to happen then it's got to be a bug in my pixel causing it and I may continue monitoring it to see what else goes missing or I'll just bite the bullet and do a fresh reset from scratch.
Side note: I do have my IPhone 7 set up with google apps so my contacts sync to it as well and the test contact synced immediately also. however, the iPhone is not losing contacts like the pixel since it does not sync to google services. I go to it to restore the missing ones LOL. I'm pretty certain the bug is on the pixel and was brought over with the March update. Got everything reset so guess I'll just sit tight for now.