API endpoint to get the GPG signature of a commit always returns 404

Summary

API endpoint to get the GPG signature of a commit always returns 404 GPG Signature Not Found even for signed and verified commits. This started happening recently for us as it worked previously. I can't tell you when exactly, unfortunately. Our current Gitlab instance is 12.8 EE.

Releated feature: gitlab-foss#49502 (closed)

Steps to reproduce

• sign a commit
• call the signature endpoint (in my case the Private-Token belongs to the root user and was given either all of the scopes, or just api and read_repository)

What is the current bug behavior?

Returns 404 GPG Signature Not Found even for signed commits.
Gitlab UI on the other hand shows the same commits as verified.

What is the expected correct behavior?

A response containing the signature details.

Relevant logs and/or screenshots

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:	2.6.5p114
Gem Version:	2.7.10
Bundler Version:1.17.3
Rake Version:	12.3.3
Redis Version:	5.0.7
Git Version:	2.24.1
Sidekiq Version:5.2.7
Go Version:	go1.13.6 linux/amd64
GitLab information
Version:	12.8.2-ee
Revision:	72ed810d281
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	PostgreSQL
DB Version:	10.12
URL:		https://gitlab..com
HTTP Clone URL:	https://gitlab..com/some-group/some-project.git
SSH Clone URL:	git@gitlab.***.com:some-group/some-project.git
Elasticsearch:	yes
Geo:		no
Using LDAP:	yes
Using Omniauth:	yes
Omniauth Providers:
GitLab Shell
Version:	11.0.0
Repository storage paths:

• default: /gitlab/repositories/repositories GitLab Shell path: /opt/gitlab/embedded/service/gitlab-shell Git: /opt/gitlab/embedded/bin/git

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 >= 11.0.0 ? ... OK (11.0.0)
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 ... 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: ... Server: ldapmain
LDAP authentication... Success
LDAP users with access to your GitLab server (only showing the first 100 results)
User output sanitized. Found 100 users of 100 limit.
Checking LDAP ... Finished
Checking GitLab App ...
Git configured correctly? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... 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? ... yes
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ...
6/1 ... yes
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285/848 ... yes
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148/850 ... yes
203/851 ... yes
148/852 ... yes
103/853 ... yes
205/854 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.5.3 ? ... yes (2.6.5)
Git version >= 2.22.0 ? ... yes (2.24.1)
Git user has default SSH configuration? ... yes
Active users: ... 147
Is authorized keys file accessible? ... yes
Elasticsearch version 5.6 - 6.x? ... yes (6.8.5)
Checking GitLab App ... Finished
Checking GitLab subtasks ... Finished

EDIT:

I should point out that I have run a sweep over all commits and all projects not older than half a year, and none of them returned a signature successfully.