I have found out that the new stick has known issues with playing certain high quality h264 files so think the issue is to do with the hardware of the stick.
Does anyone have any possible solutions?
Thanks,
Matthew
If the MediaInfo data you've posted is accurate, then on the face of it there's nothing about either of those files which should prove even remotely challenging to a Fire Stick of any generation. In fact, the bitrates are rather low for 1080p AVC files, so I doubt if the "high quality" issue you mention is a factor.
I notice that the Aliens file has a higher bitrate than Poor Things, albeit still quite low - have you ruled out possible network problems as a limiting factor? Otherwise, my guess is that the troublesome files are corrupted somehow and Kodi itself is barfing on them for this or some other reason.
Unless you're absolutely wedded to the Kodi interface(s), if I were you I'd do as suggested above and install Plex or Jellyfin server on your PC and the corresponding client on the Fire Stick, which IMO makes for a nicer user experience than Kodi/SMB in any case. If the files can't be played directly they'll be transcoded on the fly (with the reason for the transcode), although obviously that will reduce the video quality still further. If they still won't play at all, perhaps you could, errmm, acquire an alternative encoding and see if the problem persists.