I just installed an EVO 860 SSD in my MBP, and I am considering installing Mojave. Does anyone know of any issues I should...

Yes, the cable may continue to work with the original HDD. Now that you are using an SSD, you will experience increased speed and efficiency and, in turn, increased temperatures. Your SATA cable is likely already in need of replacement, but due to the much slower HDD, problems will be much harder to identify/pinpoint. The issue is directly related to the placement of the SATA cable and the MBPs' metal chassis design. I experienced identical symptoms with my MBP and replacing the SATA cable resolved everything. SATA cables are fairly cheap on Amazon (try to find one with at least a 1 year warranty). Additionally, I would recommend placing a strip of electrical tape to insulate the ribbon that comes in contact with the uppercase.