Shared Channels: @Channel mentions fail to trigger activity notifications for team-shared members

The useful distinction in your tests is direct versus team-shared membership: the same mention works for direct members, while newer channels fail for indirectly added users. Microsoft recognizes both membership types for shared channels, so I would not assume indirect members are inherently unsupported. I found no official notice establishing the backend change you suspect. Have the channel owner check mention settings, and confirm affected users’ mention notification preferences, not only “all new posts.” Ask an administrator to compare the channel’s shared teams and effective membership; Microsoft Graph’s allMembers operation includes indirect members. Reproduce with the same small test group in an older and a newer channel, using individual and channel mentions in web and desktop clients. Capture message identifiers, channel identifiers, and UTC times for Microsoft support. For urgent messages, use explicit individual mentions as a temporary measure; duplicating hundreds of memberships should not be the first workaround