The problem is still present in the latest version, I had it represent a number of times across different projects and situations, one workaround is to set the custom prop to a value on the component onStartup event before actually using it. Definitely a nasty bug needing a fix in the future
I see this issue for host in the designer. but runtime is working .
I show the value on a label and the label binding shows error in designer , however it works during runtime . not sure why . maybe a glitch in the designer. maybe worth mentioning , some minutes before that happened my colleague bound the locale to a custom prop in session . I use 8.1.44