ToolTip sometimes briefly flashes the last-shown text when appearing

* .NET Core Version:
5.0 (the project targets netcoreapp3.0, though, but I get build warnings about .NET 5 assemblies, so no idea which one it uses in actuality)

* Have you experienced this same bug with .NET Framework?:
Yes

**Problem description:**

When a tooltip instance has shown a tooltip, sometimes when showing the tooltip in another location (triggered via ToolTip.Show) or even on another control, the previous tooltip contents are briefly shown (for a single frame, perhaps; can sometimes only be seen clearly in slow motion), before the actually-intended text pops in (and also resizing the tooltip in the process, if necessary).

Toolbar, with different tooltips for different controls:

https://user-images.githubusercontent.com/1692688/114833131-8b8d0c80-9dcf-11eb-89f8-22229f795928.mp4

Custom component which uses ToolTip.Show with different texts programmatically:

https://user-images.githubusercontent.com/1692688/114833135-8cbe3980-9dcf-11eb-8053-3b41513303c2.mp4

**Expected behavior:**

The new tooltip should show up with the correct content from the start.

**Minimal repro:**

Can be as simple as having two buttons with different tooltips.