Kibana version:
7.16+

Describe the bug:
This bug is more obvious when you add a TSVB and a Lens chart on a dahboard. If you hover over TSVB, the hover line is not visible but you will see that the it is synchronized with the Lens chart. If you hover over Lens and check TSVB you will see no indication of the cursor synchronization.

I think that it works but the line is invisible for some reason. In 7.15 it looked like:

but in 7.16 (same dashboard)

Steps to reproduce:

1. Create a new dashboard
2. Add a Lens timeseries chart
3. Add a TSVB timeseries chart or a timelion agg-based chart
4. Hover over Lens, no indication of the synchronization on TSVB