When installing Haiku to an existing partition using GPT layout, there is an issue with the GTP UUID partition code which can prevent Haiku Boot EFI from identifying a Haiku partition. See this post for a way to resolve the issue:
When installing Haiku to an existing partition using GPT layout, there is an issue with the GTP UUID partition code which can prevent Haiku Boot EFI from identifying a Haiku partition. See this post for a way to resolve the issue: