I tried to follow your code and made a sample project (you can find it attached). On my side it works as expected, but I might be missing some important part of your implementation (some custom style for instance). You can take a look at the project and try to modify it in order to reproduce the issue. Then you can write us back with more information and code, so that we can investigate further why the group names are not displayed.
Thank you for your cooperation.
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