Hi, Am having this exact same problem. P3Dv4.4; PMDG 777 for P3D; FSUIPC 5.15 (registered); Linda 3.0.9 v2; Linda module PMDG 777 v1.4.
Have been away from FS for awhile, came back and went from P3D v4.2 to 4.4 doing a clean install. It used to work under P3Dv4.2, but now everything is a new release and nada. Have the MCP-II combo for Boeing. Everything else works (EFIS and MCP) except the COM section. I do program the USER buttons on the COM panel and they work properly. Have been down this road before but memory of what I did to fix it is gone. When you've dealt with flight sim issues for 40 years (FS for TRS-80), one forgets. Did the jadolfo get a resolution? Am happy to provide the usual documentation, or any that you may request. Thought I'd start here and hope there is a simple solution. For the record, LINDA is awesome, makes life so much easier.
Thanks so much,
George