Thats ShowBiz! Needless to say, after hearing that, and it wasn't a joke, that this particular Designer would never be a factor in my success. As a Set Designer, I felt extremely reluctant to perform my best, or go the extra mile when my Leader was clearly not a Team Player. I was not born to enable the success of others. I was born to be a part of it. This Designer just lost a key player.
Ideas that you have, while working on a Production, nonetheless, don't really belong to you. If you are an ideas person at all, and you need to be if you are intending to be a Production Designer, you will have your good ideas stolen.
The first time this happened to me, I was put off, however, I really admired and respected the Designer who had claimed my idea was his. The whole show he went on about it. I ate my humble pie, and learned to take it in stride. Months later, I watched 'The making of" and the Director was claiming it was his idea! No mention of the Designer.
If you have good ideas and others take them, take it as a complement. Take it as a sign that you have a gift. Use it, let it out, let others take them. After a while if it is a pattern, no matter how many ideas you had taken from you, you will eventually get found out, and you will become a go-to person on the team.
A true ideas person has a neverending spring of ideas. And no one can take the spring except you.