The latest update for Chromecast seems to have fixed the issue for me, I now see when Chromecasts transcode (the cosmetic issue) and Playback merging is not an issue anymore. Logs now show Chromecast sessions having unique IDs rather than the undefined ID shown before.
Oct 30, 2019 18:14:33.512 [11684] DEBUG - Statistics: (1ghcphkg949xhcdup79s9eh5) Reporting active playback in state 1 of type 4 (scrobble: 0) for account 1
vs
Oct 01, 2019 22:52:06.572 [2600] DEBUG - Client [undefined] reporting timeline state paused, progress of 513000/1733000ms for guid=, ratingKey=26800 url=, key=/library/metadata/26800, containerKey=, metadataId=26800, source=
