Why do contacts keep disappearing from my phone/Google?

[AC Question]

I have a Galaxy S6, and for the past few months, contacts are disappearing from my phone. I've even re-added them, only to have them disappear later.
They sync with google, and the contacts app on my phone is set to customized.

Any help would be appreciated, before I pull all my hair out.

Thanks

 

[Sergiusz Jasper]

I say save all to sim card and only have it display as sim contacts but also have a Gmail backup

 

[lucianus_luciferus]

when you create a new contact where do you save it?

 

[bgoldlist]

My contacts are saved and backed up in Gmail.
I just started saving new contacts to both phone and sim card.

What can I do about the other 400+ contacts so they don't disappear.