Mongodb cluster onprem upgrade and indexes

I have a mongodb cluster installed on my own Linux servers (version 4.0).

If I do the system upgrade of the MongoDB cluster to versions 4.2 then 4.4 then 5 then 6.0 i see that some indexes have been renamed.

Example some of them with name index_country_1 are renamed to index_country.

Is there a reindexation during the system upgrade? Why can explain this renaming?
Is it normal?

Thank you for any help you can give to me.