The first benefit of the Sentinel feat says (PHB, p. 169-170):

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

If a character with the Sentinel feat hits a creature, but that creature then gains a new type of speed (e.g. a fly speed), does that new speed also become 0 for the rest of the turn?

For example, say that a spellcaster is fighting a character who has the Sentinel feat.
It is currently the spellcaster's turn. The spellcaster's movement provokes an opportunity attack from the Sentinel character, and it hits, causing the spellcaster's speed to become 0 for the rest of the turn. In turn, the spellcaster then casts the fly spell on themselves, and thus gains a fly speed of 60 feet (a new type of movement for the creature):

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.

What is the spellcaster's fly speed (immediately after casting fly)?
Does it also become 0 for the rest of the turn? Or does the spellcaster now have a fly speed of 60 feet that they can use this turn?

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. Is my conclusion correct?