Let's suppose your Plasma application launcher has a default set of tabs or, at least, you have more than one tab configured. You choose some tab (not the first) and close launcher clicking somewhere else on your desktop.
After that, as I understand, reopening of the launcher can look a bit different on different systems:

1. It can start with the first tab opened straight away. Example found on Youtube (launcher is being opened two times between 13:47 and 14:01).
2. It can start with the tab that was opened before closing and then instantly switch to the first tab. If animations are not turned off user can see it as a fast blinking. Example found on Youtube (animation visible when launcher is reopened on 6:03, slow down to 0,5x if needed).

For the record, I have variant 2 on openSUSE Leap 15.1.
I am curious what does it depend on? Some Plasma settings, distro-specific stuff? Maybe one of the two variants (possibly 2) is a bug?

# 2 is from Plasma 5.16 beta, and is probably is not the expected action.
#1 is the expected action, iirc - the menu opens the Left tab always. This is Plasma 5.14, I think.

On my system , (Plasma 5.15.5, Qt 5.12, KDE Neon, Intel GPU) , you can *almost* catch it sliding back to the left tab, it is extremely fast, though.
I imagine the speed that it slides back in the example here may be affected by the recording software used to grab the screen, as well as the opengl rendering used in the compositor.

I wonder if this is a bug?

On my Kubuntu 19.04 system (Plasma 5.15.4, Qt 5.12.2, AMD graphics), I cannot see this at all.

I opened a bug, let's see what happens.
https://bugs.kde.org/show_bug.cgi?id=408313

If someone is interested please try to reproduce and post your findings on the bug tracker.