The GPS signal in JASSM was dropping out as you well know (or should know), due to evasive manoeuvres the missile was required to conduct in the terminal phase.

Whether Harpoon Block II is required to conduct such evasive manoeuvres or not is a matter of debate.

What is not debatable is whether or no it possesses a “land attack” capability.

JDAM does.

Clearly Harpoon Block II does, too…

It also had to do with the nature of the missile itself. JASSM is a stealthy missile where Harpoon is not. That means the GPS antenna on JASSM is likely in some funky configuration to mask it from most angles (consequently making in blind from most angles) whereas the antenna on Harpoon could be optimized for clear reception regardless of the angle.