I know in Grafana v7 (at least the early versions, maybe still now) the new table panel vertical scrollbar could be hidden if the table had a horizontal scrollbar - e.g. you had to scroll the table all the way to the right to see the vertical scrollbar.
I think there is some limit on the new table panel, see here:
https://community.grafana.com/t/show-more-than-500-rows-in-grafana-table/27790/3
Or if the new table panel is no good for your needs you can continue to use the old (v6) table panel in v7. just change the panel JSON to “type”: “table-old”