B747-400 flight crew reported flight mode did not change as expect. Flight crew overrode automation and continued approach.

Narrative

On approach into ZZZ we were experiencing a lot of GPS disruption. This was expected; but it was an added complication. While on approach we were given an expedited descent; stepping down several times to a final altitude assignment of approximately 4500ft. I was in FLCH and my Flight Mode Annunciators (FMAs) showed 'HOLD' in the Autothrottle (A/T) column. This is normal and expected. However; the plane leveled off at the selected altitude; but did not shift to Speed/Heading (SPD/HDG) SEL/ALT and remained in HOLD and FLCH. This caused the speed to drop below the selected speed; but did not drop into the yellow or red band and no other warning or alert was activated. I noticed the speed was incorrect after a couple seconds and quickly moved the throttles up and selected the next flap setting. Cause: I understand what happened and followed the FMAs. I just am not sure what causes the FLCH mode to not switch to SPD/ALT; but still level off at the desired altitude. It is something that neither the Captain nor I have seen before.