Photoshop: Banding in images (not present in Lightroom)

Hi,I'm a photographer who has been working with photoshop and lightroom for years, last year upgrading to CC (currently running the latest versions of lightroom and photoshop).I manage all my photography through lightroom, opening them in photoshop as and when required. On a recent shoot (of twilight long exposures on the Thames) I wanted to do some luminosity masking and so exported the files into Photoshop, only for them to have very noticeable banding (which is not present in the lightroom file). I have been making edits to the file in photoshop and monitoring the automatically updated image in lightroom. The banding persists in Photoshop but the gradations are perfectly smooth in lightroom. I have been exploring online for solutions or reasons for the occurrence but can't understand why these bands are suddenly appearing in photoshop, yet not present in lightroom. Nor have I found a solution (tried changing performance aspects, e.g. cache, turning off graphics processor; changing the colour settings to aRGB or sRGB; turning off profile corrections in lightroom prior to exporting to photoshop). All suggestions found online, none have worked. In terms of technical aspects: - I work primarily on a Macbook Air (2014 model) - The colourspace in photoshop is set to ProPhoto - Files are set to export as TIFF into photoshop - My monitor is colour calibrated (using a Spyder) Any ideas welcomed. Thanks.