I came across both usages but can't really tell when one is used over the other or discern any subtle difference. Is it a question of American or British English? "React on" seems certainly much lesser used than "react to" (see Youtube & Co.). Could someone maybe give some more examples, in case it's just about common phrases like in a figure of speech? Dictionaries only seem to know the latter   . Somewhat related, this about whether it is also possible to use at. Answers here indicate that one can only use "to" in combination with "react". Does that mean "on" is always wrong?
We react to a stimulus or situation.
The dog reacted to the bell by salivating.
The preposition on would not head a phrase which refers to that which is being reacted to; it might be adverbial, for example:
The emergency team can react on a moment's notice.
The dog did not react on the first experiment.
Or on could refer to something which is affected:
These chemicals react on plastics.
From a grammatical perspective, the plastics are not the stimulus. There is a reaction when the chemicals are applied to plastics.
We could say:
These chemicals act on plastics.
But people often say that the one thing reacts on the other.