Narrative:

I was the first officer flying the zzzzzy arrival at zzzzz intersection. The proper arrival was put into the FMS and all waypoints were checked for accuracy. As a new descent speed was input to the FMC; the plane increased descent to accelerate to that speed and did not level at FL240 at zzzzz intersection as desired on the VNAV profile. By the time I noticed and corrected; we had descended about 1;200 feet too low. Somehow; the crossing altitude at zzzzz had dropped out of the FMS. The captain immediately called ATC and informed them and they indicated no worries just go direct ZZZZZ1. Remainder of arrival and landing was uneventful.this was my first flight leg as pilot flying in about 3 months and the end of a long trip; so my crosscheck may not have been as sharp as I would have preferred. I'm not sure when or why the altitude at zzzzz was dropped from the FMC unless I inadvertently pushed the altitude selector knob when entering the 6;000 foot final altitude on the arrival into the altitude window. I will have to be more careful in the future about setting and double checking all altitudes at each waypoint during departure and arrival and making any needed corrections in a more timely manner.

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Original NASA ASRS Text

Title: Air carrier flight crew reported a waypoint crossing altitude dropping out of the FMS and not knowing why resulting in an altitude deviation.

Narrative: I was the FO flying the ZZZZZY arrival at ZZZZZ intersection. The proper arrival was put into the FMS and all waypoints were checked for accuracy. As a new descent speed was input to the FMC; the plane increased descent to accelerate to that speed and did not level at FL240 at ZZZZZ intersection as desired on the VNAV profile. By the time I noticed and corrected; we had descended about 1;200 feet too low. Somehow; the crossing altitude at ZZZZZ had dropped out of the FMS. The Captain immediately called ATC and informed them and they indicated no worries just go direct ZZZZZ1. Remainder of arrival and landing was uneventful.This was my first flight leg as pilot flying in about 3 months and the end of a long trip; so my crosscheck may not have been as sharp as I would have preferred. I'm not sure when or why the altitude at ZZZZZ was dropped from the FMC unless I inadvertently pushed the altitude selector knob when entering the 6;000 foot final altitude on the arrival into the altitude window. I will have to be more careful in the future about setting and double checking all altitudes at each waypoint during departure and arrival and making any needed corrections in a more timely manner.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.