That the Romans would have imported polar bears from Norway, or ...
... that Titus Calpurnius Siculus would see a white bear with his own eyes, then later write of the experience but fail to mention they were white?
If you can concede it's pretty unlikely that Calpurnius would have omitted that detail, then that opens the possibility that the Romans might have bought some Indian sun bears (which are considered more dangerous than the Eurasian brown bear) along with their Bengal tigers.
Bears in general are omnivores (emphasis on the "omni"). And even though seals are not the sun bear's natural prey , if you didn't feed them for a week, I'd say they might be inclined to nosh on the odd seal or two.
But that begs the question, how did the Romans manage to select seals for the black sun bears' opponents, when they only are the natural prey of the white bears?
Maybw they got annoyed by the seals’ barking.