Exchange 2013 Distribution list issues

Having an issue with our exchange 2013 server where we have been updating distribution lists, but the address book isn't showing all the updates.

Early today I modified a distribution list and put one of my external emails (which I created as an external contact in AD) as a member of this distribution list. Looking at the distribution list in Exchange ECP it shows me as a member. Looking in AD at that distribution list shows me as a member as well.

However, looking at that distribution list from outlook doesn't show me in there at all, and looking from OWA does not show me either.

However, I have another distribution list that in AD that has 3 users as a member of it. Exchange ECP shows those 3, and AD shows those 3 as members. However, OWA only shows 1 of those users as a member, and outlook shows 2 of those users member.

It's like we have some sort of communication issue, but everything seems to be working fine with the exception of these distribution list updates. We recently migrated from 2010 to 2013 exchange and we're only running 1 server. The other 2010 server has been retired and is no longer in the picture.