Imported gitlab issues don't retain related merge request associations

Summary

Issues imported from another gitlab instance loose relations to merge requests

Steps to reproduce

1. Create group with issue and merge request that mentions issue in it's description:
   
2. Import group with the issue
3. Check related merge requests in imported issue

What is the current bug behavior?

Imported issue is missing related merge requests section even though mentioned in merge request comment is created:

What is the expected correct behavior?

Imported issue should retain related merge requests

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:
Proxy:		no
Current User:	git
Using RVM:	no
Ruby Version:	2.7.7p221
Gem Version:	3.1.6
Bundler Version:2.3.15
Rake Version:	13.0.6
Redis Version:	6.2.7
Sidekiq Version:6.5.7
Go Version:	unknown

GitLab information
Version:	15.7.0-pre
Revision:	1d42b5f4752
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	PostgreSQL
DB Version:	13.8
URL:		http://import-target.test
HTTP Clone URL:	http://import-target.test/some-group/some-project.git
SSH Clone URL:	git@import-target.test:some-group/some-project.git
Elasticsearch:	no
Geo:		no
Using LDAP:	no
Using Omniauth:	yes
Omniauth Providers:

GitLab Shell
Version:	14.14.0
Repository storages:
- default: 	unix:/var/opt/gitlab/gitaly/gitaly.socket
GitLab Shell path:		/opt/gitlab/embedded/service/gitlab-shell

Results of GitLab application Check

Expand for output related to the GitLab application check

Checking GitLab subtasks ...

Checking GitLab Shell ...

GitLab Shell: ... GitLab Shell version >= 14.14.0 ? ... OK (14.14.0)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Internal API available: OK
Redis available via internal API: OK
gitlab-shell self-check successful

Checking GitLab Shell ... Finished

Checking Gitaly ...

Gitaly: ... default ... OK

Checking Gitaly ... Finished

Checking Sidekiq ...

Sidekiq: ... Running? ... yes
Number of Sidekiq processes (cluster/worker) ... 1/1

Checking Sidekiq ... Finished

Checking Incoming Email ...

Incoming Email: ... Reply by email is disabled in config/gitlab.yml

Checking Incoming Email ... Finished

Checking LDAP ...

LDAP: ... LDAP is disabled in config/gitlab.yml

Checking LDAP ... Finished

Checking GitLab App ...

Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Cable config exists? ... yes
Resque config exists? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... skipped (no tmp uploads folder yet)
Systemd unit files or init script exist? ... skipped (omnibus-gitlab has neither init script nor systemd units)
Systemd unit files or init script up-to-date? ... skipped (omnibus-gitlab has neither init script nor systemd units)
Projects have namespace: ...
2/1 ... yes
6/2 ... yes
10/3 ... yes
15/4 ... yes
18/5 ... yes
21/6 ... yes
23/7 ... yes
26/8 ... yes
Redis version >= 6.0.0? ... yes
Ruby version >= 2.7.2 ? ... yes (2.7.7)
Git user has default SSH configuration? ... yes
Active users: ... 8
Is authorized keys file accessible? ... yes
GitLab configured to store new projects in hashed storage? ... yes
All projects are in hashed storage? ... yes
Elasticsearch version 7.x-8.x or OpenSearch version 1.x ... skipped (Advanced Search is disabled)

Checking GitLab App ... Finished


Checking GitLab subtasks ... Finished