Air carrier First Officer reported the FMC dropped 4 navigational fixes in the routing; causing a course deviation. The First Officer stated the aircraft was in an area of reported GPS jamming.

Narrative

We had loaded; verified and both checked that the Departure SID was loaded in the FMS and verified against our ATC clearance even before pushback. We again verified the departure SID as part of navigation brief as part of the before T/O checklist. We departed ZZZ on a 160 heading. Shortly thereafter while climbing through approximately 15;000 ft MSL we were cleared direct to the ZZZZZ fix on the Departure. We selected that fix for direct to and executed it in the FMS. We selected LMAV and verified it on the FMA (Flight Mode Annunciator). The aircraft turned and flew to ZZZZZ in a climb while on the autopilot. We were switched from ZZZ1 Departure to ZZZ2 Center enroute to ZZZZZ in climb. After being cleared to climb to FL230 ZZZ2 [Center] asked us to verify what fix we were direct to at that time. We both reviewed the LEGS page immediately and discovered that the FMS had dropped 4 NAV points on the SID after the ZZZZZ fix. The FMS had taken us to ZZZZZ and reverted to heading at some point. The 4 NAV points after ZZZZZ that were dropped were ZZZZZ1; ZZZZZ2; ZZZZZ3 and ZZZZZ4. There was no annunciation that the aircraft had reverted to Heading Mode nor that the NAV points had been dropped from the FMS. Both GPS's appeared normal; there was no traffic conflict reported by ATC. There were no indications of IRS (Inertial Reference System) issues apparent. We were flying near where GPS jamming occurs sometimes but there were no NOTAMS on jamming. We were given a 170 vector for a brief time and then given direct to ZZZ1 on the SID which was still in the FMS as part of the Departure SID. We continued on in our climb and finished flight to ZZZ3. As additional note we discovered that on beginning the Arrival STAR into ZZZ3 that the preloaded altitudes for the NAV point ZZZZZ5 had dropped from the FMS. I had already verified the altitudes at ZZZZZ5 twice as the FO (First Officer) and the Captain had also verified the altitudes loaded in the FMS prior to descent. There was no annunciation of the altitudes for ZZZZZ5 dropping from the FMS. Luckily we found out ahead of the fix. After the flight we alerted maintenance and made a write up in the maintenance logbook. There were no more issues during the flight.