I think this is bug, but I’m not sure if it belongs to panel or hvplot. But the tabulator does not populate index columns properly with multi-indexing, normal display works fine. E.g.:
import pandas as pd
import panel as pn
import hvplot.pandas
from bokeh.sampledata.autompg import autompg
pn.extension('tabulator')
grouped = autompg.set_index(['cyl','name'])
grouped.interactive().pipe(pn.widgets.Tabulator, pagination='remote', page_size=15)
# grouped