I'm not sure with structure steel from toolbox. But it might be the same issue people have with fasteners time to time. What might happen is that Solidworks replaces your tweaked part with the original toolbox part.
I assume that you have used "save as" from the original toolbox part. Please see this thread that talks about Solidworks settings and how to make toolbox part copies to become regular parts.
Please mark this answer as correct if it helped you.