Elasticsearch version (bin/elasticsearch --version): 6.8.6 / 7.6.0

Description:
Trying to change the value of index_in_parent seems to succeed, but doesn't.
I would expect that it is working as designed that that value cannot be changed, but I think it's a bug that no error is returned when trying to change it.

Steps to Reproduce:

PUT /parenttest 

PUT /parenttest/_mapping/_doc { "properties": { "email": { "type": "keyword" }, "user" : { "type" : "nested", "include_in_parent": true, "properties": { "first" : {"type": "text" }, "last" : {"type": "text" } } } } } 

GET parenttest 

PUT /parenttest/_mapping/_doc { "properties": { "email": { "type": "keyword" }, "user" : { "type" : "nested", "include_in_parent": false, "properties": { "first" : {"type": "text" }, "last" : {"type": "text" } } } } } 

GET parenttest 

Expected behaviour:
The third PUT returns an error explaining that it's not possible to change the value of "include_in_parent"

Observed Behaviour:
None of the calls throws an error.
The second GET returns exactly the same data as the first.