Blame data present in SonarQube 7.9.1 is now missing after upgrade to 8.1

VERSIONS:

SonarQube: Version 8.1 (build 31237)
SonarQube Git Plugin: 1.9 (build 1725)
SonarGradle plugin 2.7.1

ERROR OBSERVED:

All issues discovered in our large git repository started showing up as unassigned after the upgrade to 8.1. If I click to the Issues tab looking at the scanned project, and pick “Authors”, the list is empty. This is after a scan from scratch, not a repeating scanned project. No blame data is showing up when viewing any of the code in SonarQube.

The gradle build log for the sonar task shows:

This repository references another local repository which is not supported. You can avoid borrow objects from another local repository by not using --reference or --shared when cloning it.
Missing blame information for the following files:
<a huge list of file paths>
This may lead to missing/broken features in SonarQube

I’ve run the JGIT 4.9.0 blame test on the git repo (and specific file paths that were validated to see the issue) as described here: https://docs.sonarqube.org/latest/analysis/scm-integration/. JGIT was showing the blame information.

The line of code that emits the error message in sonar-scm-git appears to look for a file .git/objects/info/alternates, but I’m not seeing that is an existing file. I’m not sure what else to check for.

STEPS TO REPRODUCE:
Just do a regular scan. Do it as a new project.

WORKAROUND
Downgrade to sonar-scm-git version 1.8.0.1574

We will soon enable again the analysis of clones referencing another local clone: https://jira.sonarsource.com/browse/SONARSCGIT-50.