I'm going to talk out of both sides of my mouth here.
@NAS This isn't a bug at all. Rather its a design choice by @dlandon to have UD conform to industry standard norms on partitioning. If you want to label something a bug, then the "bug" lies squarely on LT's shoulder's since they are not conforming to industry norms, and unRaid will trash a drive formatted outside of the array if partitioning isn't what it expects (regardless if a parity drive is installed or not)
That being said, this does have to get changed. With UD now being slated for inclusion in the base OS, a reasonable expectation is that the partitioning scheme on a format should match the array so that the drive doesn't get repartitioned. The better solution however is to have the OS recognize and not repartition anything that may not conform to its standards, and allow the drive to be added to the array (when parity isn't present) without losing all the data on it.