Based on your configuration, this is behaving as indented.

The Related Field Definition (zu deutsch Bezugsfeld) is meant to define a direct 'link' between two fields on different models to have exactly the same value in both (das, was wir gemeinhin als 'durchschleifen' bezeichnen).

What you need is the Compute field (zu deutsch Berechnen), in order to only set the value of x_studio_teil_1 upon selecting/changing a Sale Order Template.

Something like this should do:

for record in self:
    record['x_studio_teil_1'] = record.sale_order_template_id.x_studio_teil_1

For this to work you need to set the Dependencies field (Abhängigkeiten) to 

sale_order_template_id

in order to have Odoo know, under what circumstance to change the value.

Finally, since by default, a field with a compute function is Readonly, you may want to untick that checkbox then (Nur Lesen) so you can also override the value in the Sale Order.

This will, whenever you change the Sale Order Template on a Sale Order, reset the Value to the one from the selected Template.

If you prefer the value to remain the same if set once, change the compute method to something like this:

for record in self:
    record['x_studio_teil_1'] = record.sale_order_template_id.x_studio_teil_1 if not record.x_studio_teil_1 else record.x_studio_teil_1