Good evening everyone,
It seems to me to have come across a big bug in KDE!
Distrib: opensuse leap version 42.2
The problem is at the console (Konsole xterm).
version 16.12.1
KDE frameworks 5.26.0
The 'clear' command instead of simply generating a new page, deletes all
the previous pages and leaves only the last x lines of the format of the console.
The config of the 1000-line in console remains effective until a 'clear' is started.
To check this I use the following command:

$> for i in `seq 1 1 400`; do echo $i;x=$((i % 100));if [ $x = 0 ]; then echo "AH";clear;fi;done

- in this case all the pages preceding the last 'clear' are eliminated, in fact only
the last x lines corresponding to the format of the screen.
- by removing the 'clear' command you keep the history of the pages up to the defined limit
in the console config.
This is extremely annoying and makes it virtually impossible to write and trace a program.
To bypass this bug it is necessary to send all the tracing tags as well as the returns of the program
to a file on disk!
Impratiquable on a somewhat complex prog ...
Do you have the same behavior on your machine?

[edit]
This is probably a specific problem with the Leap 42.2 + KDE5 or maybe Bash?
Anyway I have the solution: I replace in my scripts the "clear" of bash by a 'tput clear 67 1' of terminfo.
And everything becomes normal again...

Not sure if I understand the description. For me clear just clears the page, and scrolling up still works as before. View -> Clear Scrollback and Reset deletes everything. So far this seems correct ...?

I however know the issue of the scrollback length sometimes not being changed or saved correctly ...

@ scummos
Thanks for your return, it is probably a problem specific to the Leap 42.2 + KDE5
I found the solution: I replace in my scripts the 'clear' of bash by a 'tput 67 1' of terminfo
And everything becomes normal again. )