I got a brief black screen with white inscriptions

Hello everyone.
I am a new user of brave on android since 48h and two-three times it happened to me when I change the web page to see an image containing a very brief black screen with two lines of white inscriptions on it. Probably letters but I can’t be sure, it’s really fast.

Can anyone tell me what it is?

I have brave set to be secure but I still accept cookies (except third party) and javascript. Reluctantly but well…