Incorrect creation of extended PDF

[acampove]

Bugs Please fill the below form if possible.

When creating an extended PDF, the normalization and observable range seem to be changed

Current Behaviour

I would expect only the yield to be attached to the PDF, instead the normalization range seems to also be changed

import zfit

obs = zfit.Space('x', limits=(-10, 10))
nrm = zfit.Space('x', limits=(  0, 10))

mu     = zfit.Parameter('mu', 0.0)
sigma  = zfit.Parameter('sigma', 1.0)
yield_ = zfit.Parameter('yield', 1000.0)
gauss  = zfit.pdf.Gauss(mu=mu, sigma=sigma, obs=obs, norm=nrm)

print(gauss.space)
print(gauss.norm )

gauss_extended = gauss.create_extended(yield_)

print(gauss_extended.space)
print(gauss_extended.norm )

Expected Behaviour

The PDF should be the same, except for the normalization

Context (Environment)

• zfit version: 0.28
• Python version: 3.13
• Are you using conda, pipenv, etc? : micromamba
• Operating System: Almalinux9
• Tensorflow version: 2.20.0

Possible Solution/Implementation

set_yield is a workaround, but the behavior is wrong and will give the wrong results in the user code.