737NG Common VNAV - direct to followed by speed jumping to FLAPS speed

[DirectVELAG]

I've noticed that in some specific circumstances, the FMC behaves in a strange way regarding speed.

For example, you are descending at 250kts following the STAR. You get given a DIRECT TO another waypoint on the approach, but when executed, the target speed will jump straight to FLAPS (UP speed) and the DECEL point will be removed. You are then left at whatever speed you were doing (250kts) with no shallower path angle in which to decelerate. This tends to be when the distance to this new waypoint is greater than say 50nm, and if executed before reaching the original DECEL point.

I'm thinking this has something to do with either a) approach logic, or b) the logic with cruise descent if initiated more than xx miles before TOD (80nm rings a bell)?

Can anyone shed any light?

Thanks