If a file already imported into LR CC 2015 is changed in an external editor the Library Module will recognise those changes but the Develop Module fails to do so and simply shows the original file. This is not how LR 5.7 behaved.
To explain - I import into LR 16 bit Tiffs from my Sigma DPxM cameras processed in Sigma Photo Pro. If, however, I go back to Photo Pro and re-develop the Raw files and export them as 16bit Tiffs (and thus overwriting the original file) the Library Module shows the changes to the file but the Develop Module does not - it shows the original (now overwritten and obsolete) file. I have to quite LR and restart in order to see the changes. In LR 5.7 and earlier simply zooming into the file in the Develop Module forced LR to read and display the new file. Is this a bug? Is there any way to force LR to re-read the underlying file after it has been changed?
Would suggest you to go under Lightroom Preferences > Performance and uncheck the option to use Graphics Processor.
Then, restart Lighroom and try to reproduce the issue.