Merge requests from old deleted projects do not have "merged_at" date in API

Summary

Trying to get a merged_at date from a merged MR, I get null, while the web UI shows a correct merge date. This happens for a few MRs that have had their source projects removed, I'm not sure if it happens for all such MRs.

Steps to reproduce

1. Visit Molcas/OpenMolcas!220 (merged)
2. Note "Merged by MargePT2 3 years ago (Jan 15, 2020 7:31pm GMT+0100)" and "The source project of this merge request has been removed."
3. Run curl "https://gitlab.com/api/v4/projects/Molcas%2FOpenMolcas/merge_requests/220" | json_pp | grep merged_at
4. Note "merged_at" : null

Example Project

Molcas/OpenMolcas!220 (merged)

What is the current bug behavior?

No usable merged_at date with API (and merged_by, and possibly other data).

What is the expected correct behavior?

Since the merge date is available, I'd expect to get it with the API too.

Relevant logs and/or screenshots

$ curl "https://gitlab.com/api/v4/projects/Molcas%2FOpenMolcas/merge_requests/220" | json_pp  | grep merged_at
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  2694    0  2694    0     0   3345      0 --:--:-- --:--:-- --:--:--  3342
   "merged_at" : null,

Output of checks

This bug happens on GitLab.com