I don't know if the Power Apps team did something since then, but I am able now to set default value for managed metadata fields.
Here is how to do it...
When you insert a datacard in your form for a managed metadata field, the Items value for the dropdown list is as follow:
Choices([@Bills].scVehicule)
So, if you want, let's say, to set a default value to your managed metadata field, do it by putting a formula like this one in the DefaultSelectedItems property of the dropdown list: