Anyone had an issue requesting a Release Preview instance for their Sandbox account?

you cannot have release preview for sandbox, only for prod

Netsuite Support say otherwise, but I'm not convinced.

As an alternative you can consider creating Release Preview account from Sandbox 1234567_SB1 to avoid your 3rd party configurations interacting with our Production systems. You will need to opt-in in Sandbox via Setup > Company > Release Preview > Click Request Release Preview.

All the same I'm a bit nervous to request it. (btw that Netsuite response was from a support case I logged)