It's a bit of an edge case, but for example:

math.config({number: 'BigNumber'})
math.add('2', '3')
// results in a number with value 5, but should be a BigNumber with value 5

The reason is that functions like add do not have a signature like add(string, string). Therefore, automatic conversion kicks in. The automatic conversion finds a conversion from string -> number that fits the bill and uses that, without reconing with the configured config.number. I'm not sure how a proper solution to fix this would look like, since the type conversions are static 🤔.