Gaining a new type of movement speed after being hit by a Sentinel OA. New speed = 0?

In the following example, what is the spellcaster’s fly speed?

Example:

  • A character with the Sentinel feat, successfully makes an Opportunity Attack against a spellcaster creature.
  • The spellcaster in turn, casts the fly spell on themselves. Gaining a fly speed of 60. A new type of movement for the creature.

Sentinel:

When you hit a creature with an opportunity attack, the creature’s speed becomes 0 for the rest of the turn.

Fly:

You touch a willing creature. The target gains a flying speed of 60 feet for the duration. When the spell ends, the target falls if it is still aloft, unless it can stop the fall.

From my understanding, there isn’t a so called "stack" like there is in Magic: the Gathering. The effect from Sentinel is very specific. It doesn’t matter if the creature gains a new type of speed after the sentinel effect has been applied. All of its speeds are 0 for the rest of the turn.