Highlight doesn't work when series ID is a number. This is because we use the series ID as a key in the series object here.

Keys in object cannot be numbers and will automatically be coerced to string:

Each property name is a string or a Symbol. Any other value, including a number, is coerced to a string. This outputs 'value', since 1 is coerced into '1'.

Source

const a = { 1: 'a' };
console.log(a['1']); // a

Reproduction

Here.

The second series' ID is 1 instead of "1", so highlight doesn't work. If you change it to a string, then it works.

Alternatives

1. Restrict series IDs to strings. This way, we can keep using objects for storing series. I think this might be best because data-series doesn't distinguish between strings and numbers, so we'll have issues there as well.
2. Use a Map to store series. This way, we can keep using numbers for keys, but it would break all use*SeriesContext because it is exported and accessing maps using the bracket notation doesn't work.