For more information about that channel's features and functionality, you'll want to contact PLEX support directly to inquire further. Many channels on Roku are developed and maintained by the channel provider themselves.
You are no doubt correct, but at the end of the day perhaps in the interest of customer satisfaction, Roku might want to contact PLEX about their app. The reason we purchase Roku's is for specific capabilities. When those capabilities fail, customers look around at the competition. I have 2 other brand products, similar to your product, without this Plex issue.
In fact, the current Plex app on Roku now doesn't support the Plex server version I have on my brand new NAS, and there's no way to update it on the NAS until Plex provides a new update. But, if I use your competitor devices, I have full Plex capability. That's not good for Roku, regardless of who is to blame.
I know the integration fuxed it for some, my issue was somewhat different. I had all local channels on both apple and amazon, regardless of how many times I deleted and reset the tuner, but with roku it was greyed out. Turns out it's pretty picky about which server it gets the channels from, I have two, only one has a tuner because it's just easier to keep track of. There were two entries for the same server, I unchecked one and checked the other and it worked, no idea why there were two.