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Hello and thanks.

If this is a bug, then please add a change log; see https://github.com/astropy/astropy/blob/main/docs/changes/README.rst

Also would need a regression test demonstrating that this bug would cause a failure on main but not with your patch.

I've added a changelog and a test showing the bug and then the patch fixing it.

What I added to test_fitters.TestJointFitter:

1. The JointFitter edits the model parameter values in place rather than returning a copy of the models altered; therefore, I had to initialise another couple of models (self.g3=models.Gaussian1D(amplitude=10, mean=14.9, stddev=0.3) and self.g4=models.Gaussian1D(amplitude=10, mean=13, stddev=0.4)) to pass to the new JointFitter in TestJointFitter().setup_class().

2. I then added a new JointFitter instance called self.jf2 (since the first one is called to fit straight away and to preserve older test behaviour), also in TestJointFitter().setup_class().

3. Finally, I created a new method called test_initial_parameter_values that will check the values JointFitter uses as the initial values for all of the parameters to be fitted.

The two behaviours are:

1. In main, test_initial_parameter_values fails as the initial values from self.fitparams should be [ 9.8, 14.0, 0.3, 0.4] but instead gets[9.8, 14.0, 14.9, 13.0] as the bug has been described.

2. The patch I've suggest here passes this test and all other existing TestJointFitter tests.

Apologies @pllim , I forgot to tag you once I had attempting to take your recommendations into account. Please let me know if there is anything else required in the PR or if the test I've added is not adequate and needs improved.