I've got a relatively new RDS 2019 environment setup which has been active for around 2 months with a handful of users for testing. The image seems to work for the session hosts so I've started to add more users and server at a time I've been adding in a new Session Host to increase capacity. I've done this 3 times so far, once for the initial test 'host0' then another twice to add in extra capacity 'host1' and 'host2'. I've also built a 'test' collection with 'testhost0' session host as a play ground for tweaks before mirroring on live. All this worked just fine.
Last week I tried to build another server and add to a new collection for a new group of users which will have a much more customised desktop so using the same process as the all the other hosts (I'm using a SCCM task sequence to deploy defined image and additional software), then moving the computer object in AD into the RDS OU so it gets the right GPO's etc. The build process all works exactly like the ones in the past and everything looks to be working until it comes to adding into a collection. I've taken the following screen grabs to show whats happening and this is where I'm a bit lost..
In the following screengrabs, RDSHOST3 is the 'new' VM I'm trying to create a new collection for. so....
1) After building server, Add to All Servers in Server Management.

2) Add new Session Host. Server is then seen in Deployment Servers list

3) At this point if I look in RDS Management under Servers, I can see that its not listed.

4) But its listed under Host Servers?

5) So I try to add into a collection and I get an error

I've had a look though the event logs on the Session Host and on the Brokers but can seem to locate anything which helps. I've also gone a bit of searching forums etc but I've yet to come up with anything which point to an actual 'cause' which I can look to fix so being fairly new to RDS and having not been running 'live' very long I don't want to poke around too much without finding some direction.
Is there anything I can get which would help with diagnostics?
Is this an error of sorts or I'm I doing something wrong in my process to add a Session Host?
Cheers