Using iPhone 7+ 4Kp30 footage on a HD timeline in FCPX

I have some footage that was shot using an iPhone 7+, in 4K at 30fps. The footage is smooth and sharp on the phone, and after import to the latest version of FCPX.

I need to combine this with some 4K footage taken on a Sony AX700 at 25fps, which is where my problem starts. When I pull both onto the timeline, the iPhone footage becomes very choppy (jerky) on playback and not smooth. The same effect is there when rendered out to a ProRes master file. The timeline is 4K 25p.

Just to check, I've copied the entire timeline on to a new 4K30p project and the iPhone footage then plays fine, but everything else stutters every 60 odd frames - as if there is an extra frame being inserted - which I suspect is to be expected given the higher frame rate of the timeline. I have tried using Compressor to output the iPhone footage at 4K25p Prores 422, but get the same result, so it is rebasing the iPhone video that is causing the issue.

The best option I have currently is to run the iPhone video under 'Automatic Speed' on the timeline, which is basically playing it at 80% (its choice of figure) which deals with probably 70-80% of the problem, but makes the shot look a little interlaced.

I am assuming I cannot be the only one to hit this problem, so can anyone advise what is the best way to incorporate iPhone 4K30p footage into a lower frame rate timeline to get smooth playback?

Rich