GitHub PR comment threads not imported correctly

Summary

In using the import from GitHub, comment threads are not correctly imported -- the behaviour seems nondeterministic.

Steps to reproduce

We are finding this on all projects we import (projects are private in GitHub) that have comment threads in PRs.

What is the current bug behavior?

Here's an example comment thread in GitHub

In one import to GitLab it imports as this set of multiple comments ('Overview' screen)

On another import, it still doesn't associate the comment thread, but is different.

Similarly, the 'Changes' screen produces unpredictable results for the comment threading.

What is the expected correct behavior?

The comment threads should be similar to the GitHub threads.

Relevant logs and/or screenshots

Included in the description.

Maybe related to #336596 (closed)

Output of checks

This bug happens on GitLab.com