Having a user as myusr@myrealm and a relative principal myusr/myinstance@myrealm, the vault owned by the user is not accessible using the principal myusr/myinstance@myrealm.
In order to let the session using the principal myusr/@myrealm, it is necessary to share the container suing ipa vault-add-member, but the share doesn't work if the session on the instance is not alive or it just disappears when the session is closed.
Every time a user comes with whatever principal user/[instance]@realm it should see its vault containers.
$ rpm -q freeipa-server freeipa-client ipa-server ipa-client 389-ds-base pki-ca krb5-server
package freeipa-server is not installed package freeipa-client is not installed ipa-server-4.9.11-7.module+el8.8.0+19639+24a8b95c.x86_64 ipa-client-4.9.11-7.module+el8.8.0+19639+24a8b95c.x86_64 389-ds-base-1.4.3.35-2.module+el8.8.0+19693+b24f535c.x86_64 package pki-ca is not installed krb5-server-1.18.2-25.el8_8.x86_64
FreeIPA does not have any support for relative principals. The behavior you see is correct.
Vault API has concept of ownership. You can add additional owners and members with ipa vault-add-[owner|member] commands. See more documentation with ipa help vault.
ipa vault-add-[owner|member]
ipa help vault
Metadata Update from @abbra: - Issue close_status updated to: invalid - Issue status updated to: Closed (was: Open)