Option group_label does not work when plots are overlaid

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#### ALL software version info
```
hvplot 0.10.0 pyhd8ed1ab_0 conda-forge
holoviews 1.18.3 pyhd8ed1ab_0 conda-forge
bokeh 3.4.1 pyhd8ed1ab_0 conda-forge
```
#### Description of expected behavior and the observed behavior
One can specify legend title with option ``group_label``. However, the title disappears when 2 or more plots are overlaid. In the example below, ``p1`` and ``p2`` display the title, ``p1 * p2 `` does not.
#### Complete, minimal, self-contained example code that reproduces the issue

```
# code goes here between backticks
df = pd.DataFrame([[0.2, 0.7], [0.8, 0.3]], index=[0, 1], columns=['a', 'b'])
p1 = df.hvplot.scatter(group_label='column')
p2 = df.hvplot.line(group_label='column')
p1 * p2
```

#### Stack traceback and/or browser JavaScript console output

#### Screenshots or screencasts of the bug in action

- [ ] I may be interested in making a pull request to address this