I have some difficulties to reproduce this problem on my test project. My suggestion is to try to delete the problematic item and to try to create it again. If you share some detailed information about your custom content type we will get a better picture of the problem.
You can also try to upgrade a copy of your project to version 6.0 and to inspect for this problem. This problem could be specific for version 5.4.
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system and vote to affect the priority of the items