Datasheet say when Changing 3.3V→2.5V、2.5V→1.8V （ SEL_=VCC）Propagation Delay is 0.225ns .
When changing 3.3V->1.8 V （ SEL_=0V）, is Propagation Delay 0.225ns?
Yes, propagation delay should be 0.225ns.
Also, kindly have a look at end note 4 which states that the digital switch contributes no propagation delay other than the RC delay of the typical R ON of the switch and the load capacitance when driven by an ideal voltage source. Since the time constant is much smaller than the rise/fall times of typical driving signals, it adds very little propagation delay to the system. Propagation delay of the digital switch when used in a system is determined by the driving circuit on the driving side of the switch and its interaction with the load on the driven side.
Timing Measurement Information can also be found on page 8 of the ADG3247 datasheet.
Let me know if this helps.