Inconsistent tooltip title formatting for multiple-y-series tooltip configuration when only value formatter is provided

[rahulr-96]

Description

There is an inconsistency in the tooltip label behavior with tooltip that targets multiple y series when a formatter function is provided only for the tooltip value. The default formatter applied to the tooltip title differs in this case compared to when no formatter is provided at all.

Steps to Reproduce

1. Configure a tooltip with a custom formatter to multiple y series for the value only.
2. Observe the default formatting applied to the tooltip title.
3. Compare this with the tooltip behavior when no formatter is provided.

Expected Behavior

The default formatter for the tooltip title should remain consistent regardless of whether a value formatter is supplied.

Actual Behavior

The default formatter applied to the tooltip title changes when a value formatter is used without a title formatter.

Screenshots

Reproduction Link

https://codepen.io/rahulr-96/pen/dyxrQrL