@j.nimbona
From your image it looks like you need to apply some end treatments to those beams. Since Frame Generator is sketch driven, beams that are placed will initially come in intersecting the sketch line they were placed on. Using some combination of end treatments (trim/extend, miter etc), depending on how the beam was placed and where, will give your frame a more finished look and will then report accurate cut lengths for your material in your BOM.
This is a Help page from IV 2022 about End Treatments and how to apply them (it hasn't changed much if you're on a newer release), and I would also recommend spending some time on YouTube searching for Frame Gen tutorials to help you to understand the basics.
Good luck, and keep bringing those questions here, this is a great place to get help.