I have encountered this for years, and not just in Lightroom
I just updated Photoshop 2015 and Lightroom now opens a version of Photoshop on one of my "cloned" start-up drives.
Unfortunately, there use to be a simple way to "point" an Adobe app to the correct external editor. Now it appears to be done automatically, which sucks because Adobe opens the wrong version, continually. it seems to me I've gone through this in InDesign and Dreamweaver as well.
Is there a bullet-proof way to ensure that Lightroom opens the "correct" version of Photoshop which is on my start-up drive?
Using the most current versions of Lightroom and Photoshop CC.
Adobe has released a new version of Photoshop CC today and you may wish to install it to see if it helps with your issue. Quit Lightroom and don’t re-launch Lightroom until the new version of Photoshop is installed. LR should then automatically recognize the latest version of Photoshop on next launch. See my blog for further information on Photoshop CC 2015.5