Air carrier pilot reported a false terrain alert and ADS-B failing EICAS message while flying at 30;000 feet near SULOM intersection. The reporter stated they were not sure if this was the result of GPS jamming since this area is reportedly not known for that.

Narrative

Departed heading north on our standard routing at 30;000 feet. As we approached waypoint SULOM we noticed the navigation indication on the lower portion of the PFD (Primary Flight Display) was switching from GPS to INERTIAL and back. Also received the EICAS (Engine Indicating and Crew Alerting System) message - ADS B OUT R. This was a new area for navigation disruptions. Normally these types of disruptions occur [in another area]. We made contact with OPLR FIR. Half way between waypoints SULOM AND INDEK with the indications mentioned above; experienced a TERRAIN WARNING. The First Officer was the flying pilot and he reacted to the warning. We climbed about 400 ft. and the warning started to subside. Contacted OPLR to check our position. He showed our altitude at just above 30000 ft. well above the terrain. At the time of the terrain warning; I am fairly certain that there was not a position/map shift.As mentioned above; the OPLR FIR in my experience is not an area of GPS spoofing and navigation disruptions. I flew the same route the next week and did not experience any problems; GPS; ABS B; or TERR. In the areas of extreme GPS spoofing; where we experience loss of both GPS and even look ahead TERR; I have never received and TERR WARNING. I believe my first officer was correct in reacting to the TERR WARNING given the position we were in; approaching some of the highest terrain in the world. We did contact dispatch with all of the details. Maintenance also pinged the aircraft and found no major faults. This event is just part of the challenges of the ZZZ to ZZZZ route. I see no internal or external causal factors.