I've worded the question in two ways, before and after context, both bolded text below.
Let's say you have some fly speed with average maneuverability. Speed is not important. You have a minimum forward speed (MFS) of half, an up angle of 60 degrees, and an up speed of half. Imagine you begin ascending. Aside from beginning your ascent (where you have to fly steady for half maximum distance before ascending), does MFS still apply if you continue ascending the next round? If minimum forward speed applies when ascending (moving at your "up speed"), then implicitly you need a full double move to ascend at the full 60 degrees, and in general you cannot ascend at 60 degrees (math below).
For 60 degrees, the standard trig triangle has a hypotenuse of 2 and a base of 1. In D&D terms, however far you fly at 60 degrees, you fly half that distance forward. Therefore, if your up speed is half, flying half your speed at 60 degrees means you fly only one-quarter your speed forward, which doesn't meet the minimum forward speed of half requirement.
It's not hard to see that flying at 60 degrees under this logic requires a double move, and you must use all your movement to achieve the minimum forward speed (a double move at half speed is full speed along 60 degrees, which means you moved half speed forward).
Otherwise for a single move action, you're capped at much smaller angles of ascension, if that's even technically possible within the rules. For example, with a fly speed of 60 and average maneuverability, you could still move foward 30 and up 15. Again, it doesn't really say whether or not you can ascend at lower angles, but I wouldn't see why not.
So, is this implicit logic correct, or is ascension supposed to be a different mode of movement whereby minimum forward speed shouldn't apply?