The newer version is more stable and has less trouble rendering effects, but:

Behind text and also images that do not cover the screen, everything behind the text is black. This was not the case in previous KDEnlive versions.
(KDEnlive 18.08.2)

That can be bypassed by the affine/composite transitions, but that requires the video track to be right below the text.

How can I remove that black background that comes when adding a text track or any image track?

Is there any way to get transparency behind text in KDEnlive 18.08.2?

In earlier versions such as 18.04.1, I could just add text on top of video and there was no dark background.

Am I the only one to experience this issue?

I have just seen in the release calendar, that KDEnlive 18.08.3 will be released today.

I will try out 18.08.1 to see if this bug is still present.

Guess what! I just fixed it! "Track Compositing" needs to be set to “high quality” (above the upper left corner of video timeline).

Great thanks to the developer team for fixing instability bugs with KDenlive during the recent months
Effects also work. And rendering got faster.
KDEnlive is getting mature!

(I am using Linux Mint (Xfce) right now.)