To fix the bugged dig spots such as the ones in Mt. Coronet Summit, go to Oreburgh Pokecenter and talk to the Explorer NPC (a similar NPC exists in Kanto and Johto to fix the dig spots of their respective areas). He will ask if there are any "misbehaving dig spots in Kanto" (yes, in Kanto - pls fix the text!) but he'll fix the Sinnoh ones.
I was doing Sinnoh dig spots and finished my run in Oreburgh, then saw the NPC, recognized him and talked to him. He said something along the lines of "I fixed them for you!" instead of the usual "None needed to be fixed...". I immediately went back to Mt. Coronet Summit and the 2 southernmost spots (the bugged ones) were available! Now we have more chances for synchronizable Honedge :)