"abort: stream ended unexpectedly" when pushing closed branches

Summary

Closing and pushing an already merged branch is resulting in the following error

pushing to ssh://git@hg.cathica.austiger.com:2222/telligencia/psite-native
searching for changes
remote: adding changesets
remote: adding manifests
remote: adding file changes
abort: stream ended unexpectedly (got 0 bytes, expected 4)

Before upgrading to heptapod 0.17 this issue did not occur, but rolling back to 0.16 still has the same error. We've also upgraded the instance to 0.20 and have pushed to a new "native" repo and still have the same issue.

I haven't been able to reproduce the error with other repositories

Steps to reproduce

1. Close a branch that has previously been merged into default (and pushed) and then push.

What is the current bug behavior?

Mercurial has an "abort" error and does not push the single commit

What is the expected correct behavior?

The commit is pushed

Relevant logs and/or screenshots

[2021-03-23 02:36:27 +0000] [10602] [WARNING] repo:/var/opt/gitlab/git-data/repositories/@hashed/3d/91/3d914f9348c9cc0ff8a79716700b9fcd4d2f3e711608004eb8f138bcba7f14d9.hg [hg] error: pretxnclose.heptapod_sync hook raised an exception: unknown revision 'ADMIN_Christmas_Tree'
[2021-03-23 02:36:27 +0000] [10602] [INFO] repo:/var/opt/gitlab/git-data/repositories/@hashed/3d/91/3d914f9348c9cc0ff8a79716700b9fcd4d2f3e711608004eb8f138bcba7f14d9.hg [hg.pythonhook] pythonhook-pretxnclose: heptapod.hooks.gitlab_mirror.mirror finished in 45.64 seconds
[2021-03-23 02:36:27 +0000] [10602] [WARNING] repo:/var/opt/gitlab/git-data/repositories/@hashed/3d/91/3d914f9348c9cc0ff8a79716700b9fcd4d2f3e711608004eb8f138bcba7f14d9.hg [hg] transaction abort!
[2021-03-23 02:36:27 +0000] [10602] [WARNING] repo:/var/opt/gitlab/git-data/repositories/@hashed/3d/91/3d914f9348c9cc0ff8a79716700b9fcd4d2f3e711608004eb8f138bcba7f14d9.hg [hg] rollback completed
[2021-03-23 02:36:27 +0000] [10602] [ERROR] repo:None [hg] abort: unknown revision 'ADMIN_Christmas_Tree'!
[2021-03-23 02:36:27 +0000] [10602] [INFO] repo:None [hg.commandfinish] -R '/var/opt/gitlab/git-data/repositories/@hashed/3d/91/3d914f9348c9cc0ff8a79716700b9fcd4d2f3e711608004eb8f138bcba7f14d9.hg' serve --stdio exited 255 after 57.52 seconds

Details about the Heptapod instance where the bug occurs

Heptapod Docker instance running on an ubuntu 18.04.4 LTS server

Components

Heptapod 0.20.0

Mercurial 5.6.1

hg-evolve 10.2.0.post1

hg-git 0.9.0 (dulwich 0.20.19)

GitLab 13.9.1

Heptapod Shell 13.16.0

Heptapod Workhorse 0.20.0

GitLab API v4

Python 3.8.8

Ruby 2.7.2p137

Rails 6.0.3.4

PostgreSQL 11.10

Redis 6.0.10