To narrow down this one, I would confirm some information with you:
Whether this phenomenon occurs on all Exchange online distribution group? If this phenomenon only occurs on a specific distribution group. I would suggest you try to create a new one.
Whether you modify the default hybrid connector before? If you modify it before, I would suggest you try to rerun HCW to create a new one.
Which email address that you used for this distribution group? I would suggest you try to use the Exchange online default domain for this group then check whether the phenomenon is gone.
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