What can cause a mapping to be corrupted?

My mapping seems to get corrupted sometime.
A simple mapping such as:
properties: {
field1: {
index: not_analyzed
omit_norms: true
index_options: docs
type: string
_parent: {
type: parent_doctype

suddenly losses the "_parent" part after several indexing operations

I cannot recreate this, as this happens randomly, as far as I can tell.

When it happens, all the "_parent" fields of existing docs also vanish.

I most definetely do no mapping API calls that could cause this.
Only _index (with autogenereated IDs)

When can (or is it possible for) an index operation change the mapping???