Eternatus-1 can Transform only in scenarios where Dynamax is possible, and will appear as the Dynamaxed form of the target Pokemon. Otherwise the game will crash. Previous crashes were within the Pokemon cafe in a doubles battle where Dynamax is not possible.
Versus Morimoto, Eternatus-1 can Transform into his Grapploct (shows up Dynamaxed) or your ally Charizard with GMax factor (shows up as GMax Charizard).
I confirmed this behavior in local battle. Eternatus-1 Transforms without a crash into opposing Sirfetch'd, which again shows up as its Dynamaxed model.
Eternatus-1 does not need the GMax factor for this behavior. Transforming into Eternatus-1 still gives you Eternatus-0.
I've added additional clips to the end of https://imgur.com/a/MeXiTs6
, and edited my original post to clarify the new findings.