In v0.12.0 and later, the mouth of the Default expression of models made in VRoid Studio has now been changed from "slightly open" to "closed".
Basically, the parameters of the Default expression of models made in VRoid Studio v0.11.3 and earlier was set to 0, but in v0.12 and later, it'll be set to 100.
This change can be applied to a VRoid file made in v0.11.3 or earlier after updating the file in v0.12 or later. However, it's also possible to restore the mouth shape to how it was originally using the facial editor and exporting the VRM. For more information on how to restore files to their previous state, please see the User Guide.
Before (v0.11.3 and lower)
After (v0.12.0 and up)
If you want the model's expression to be as it was in v0.11.3 or earlier, please follow these instructions.
１．From the upper menu bar, please go to Face Editor > Expression > NEUTRAL.
Next, scroll down the Property Panel on the right-hand side.
２．In the "Mouth" category, move the "Default" slider to the left until it hits 0.
And that's how to restore the default of v0.11.3 and earlier.