Exchange 2010 won’t deliver a message to a DistributionGroup-Universal

this is the situation:

messages sent to a DistributionGroup-Universal group are not delivered to the group members. The same message sent to the group + a directly member address is delivered just to that member. For example, here is a example with Get-MessageTrackingLog from a message:

sent: external user

to: group@mydomain.com[member1, member2, member3, member4], member1@mydomain.com

MessageId       : <0001>
EventId         : RECEIVE
Timestamp       : 14.12.2016 19:03:47
Recipients      : member1@mydomain.com, group@mydomain.com}
RecipientStatus :

MessageId       : <0001>
EventId         : EXPAND
Timestamp       : 14.12.2016 19:03:47
Recipients      : {member1@mydomain.com, member2@mydomain.com, member3@mydomain.com, member4@mydomain.com}
RecipientStatus : 250 2.1.5 RESOLVER.GRP.Expanded; distribution list expanded

MessageId       : <0001>
EventId         : TRANSFER
Timestamp       : 14.12.2016 19:03:47
Recipients      : {member1@mydomain.com, member2@mydomain.com, member3@mydomain.com, member4@mydomain.com}
RecipientStatus : {, , , }

MessageId       : <0001>
EventId         : DELIVER
Timestamp       : 14.12.2016 19:03:47
Recipients      : member1@mydomain.com
RecipientStatus :

MessageId       : <0001>
EventId         : DUPLICATEDELIVER
Timestamp       : 14.12.2016 19:03:48
Recipients      : member1@mydomain.com
RecipientStatus :

you can see the 250 2.1.5 RESOLVER.GRP.Expanded in the EXPAND event, and later the message is delivered just to member1, which is the only one user addressed in the e-mail out of the group. I believe the DUPLICATEDELIVER event is because member1 is addressed 2 times (inside the group and directly).

Finally, the e-mail was not delivered to the group members (and the sender didn't receive any NDR) and I don't understand why (it's a DistributionGroup-Universal, not DDG)

UPDATE:
I tried again (from a different external account) and now the e-mail was delivered first to member1, and after to all group members, except member1 because is a duplicate message. I believe this is the right behavior:

MessageId       : <0002>
EventId         : RECEIVE
Timestamp       : 21.12.2016 7:05:02
Recipients      : {member1@mydomain.com, group@mydomain.com}
RecipientStatus :
MessageSubject  : test mail

MessageId       : <0002>
EventId         : EXPAND
Timestamp       : 21.12.2016 7:05:02
Recipients      : {member1@mydomain.com, member2@mydomain.com, member3@mydomain.com, member4@mydomain.com}
RecipientStatus : 250 2.1.5 RESOLVER.GRP.Expanded; distribution list expanded
MessageSubject  : test mail

MessageId       : <0002>
EventId         : TRANSFER
Timestamp       : 21.12.2016 7:05:02
Recipients      : {member1@mydomain.com, member2@mydomain.com, member3@mydomain.com, member4@mydomain.com}
RecipientStatus : {, , , }
MessageSubject  : test mail

MessageId       : <0002>
EventId         : DELIVER
Timestamp       : 21.12.2016 7:05:02
Recipients      : member1@mydomain.com
RecipientStatus :
MessageSubject  : test mail

MessageId       : <0002>
EventId         : DELIVER
Timestamp       : 21.12.2016 7:05:02
Recipients      : {member2@mydomain.com, member3@mydomain.com, member4@mydomain.com}
RecipientStatus :
MessageSubject  : test mail

MessageId       : <0002>
EventId         : DUPLICATEDELIVER
Timestamp       : 21.12.2016 7:05:02
Recipients      : member1@mydomain.com
RecipientStatus :
MessageSubject  : test mail

The original problem happened the last week, concretely from only one external address, and I see others emails to the group (from others external users) had been delivered to all group members. I tried now the combination of group + member1, how was in the problematic message for check if the noise come from the duplicate message, but I see there is delivered to everybody, so, I don't know what can be…