The only issue may be the seal. Its should 'click' in and not pull off easily, if it continues to pull out really easily, you will have to remove the cassette, then slide it on.
The wheel is all good thanks.Might just be end caps.
The knob/cap that sticks on the end of the freehub and pokes out of the cassette that the axle goes through was super floppy and loose and sticking out too far which made me think it had unthreaded somehow. Turns out it has an internal o-ring and just needed to be tapped back into place on the end of the freehub. Crisis avoided, sanity restored.