I have exactly the same problem. We have two instances connected to our 7.2 internal SonarQube (Community Edition), our TFS instance has no issues and that runs 4.2.0 version of the VSTS Build Task.
Our VSTS instance has the problem that Alpay describes and that runs the 4.3.0 version of the build task.
Reading the release notes I see that 4.3.0 contains a lot of PR fixes:
https://jira.sonarsource.com/browse/VSTS/fixforversion/14393/?selectedTab=com.atlassian.jira.plugins.jira-development-integration-plugin:release-report-tabpanel
Specifically this seems like a culprit:
https://jira.sonarsource.com/browse/VSTS-156
“If the SonarQube server supports it (SQ >=7.2) then we should pass appropriate parameter on scanner side”.
I don’t work for Sonarsource and cant see the source code so this is just a guess, but seems like the 4.3.0 version of the TFS/VSTS task doesn’t account for us Community users that don’t have the branch plugin.