Supposing that the measurements with and without the sphere are taken under identical circumstances. But you deserve the upvote either way.

Assuming the time between submersion and observation is greater than zero, increasing temperatures above boiling would give increasing error.

Conversely the physicist may get other error at/near freezing.

And yet another option is the fluid being above the Ball's melting and/or boiling point, assuming changes in density with phase change.

Interestingly, the physicist's answer would always be correct, but not always consistent.

If the sphere is compressible, then air pressure will slightly affect its volume. And unless the thermal expansion rate of the sphere is zero, the amount of fluid displaced will also vary with temperature.

However, even if the physicist gets different answers in different environments, it's always still a correct answer, because they correctly measured the volume of the sphere at that time.