Running that powershell does show the DRS and issuance transforms...but nothing about an RP (unless that IS the RP and just isn't worded that way). If that is the case then there must have been a bug that allowed it to show up as an RP trust.
It is very strange...I've been doing this a long time and I know I saw the RP trust...I'm wondering if there is a chance it was an artifact that shows up under the right conditions...I'm sure it was there because I intentionally deleted it. In fact, the chain of events was that I deployed ADFS4, got curious and enabled Device Registration thinking it would JUST be for ADFS clients, then saw that magically (and very unwanted) I had all of my domain-bound computers starting to try to register with the service - even though I didn't change any policy or configure them to, they just magically started trying to Join. I immediately started trying to undo the Device Registration enablement. The first thing I did was Disable Device Registration, which made it go amber from green, but at this point it said the forest was already prepped - and I wanted to un-prep it. Then because they kept trying to enroll spontaneously and pretty much with an ongoing flood, I deleted the RP from this totally fresh ADFS. It asked me if I wanted to delete it and I thought, yes, no way to back it up, if I want to put the RP back after more testing, I'll just reinstall ADFS...nope...gone for good. So, not sure if there was something about my domain/forest that allowed a bug to surface and create the RP and maybe completing the enablement, the first time, of the ADFS install prevented the RP from ever being recreated. Couldn't reproduce this in the test environment which tells me some VERY specific condition was met originally to result in this RP having been created or at least be exposed visibly in the GUI.