Sure, it is a flaw. But the developer still needs to make a new release for it 
~/.config/kuprc
Thanks, @claydoh. I found this file. On my system it presently look as shown below. I thought I did not find any information on the time interval, but “Accumulated usage time=2520” could, of course, be the scheduler’s interpretation of my choice “1 hour”. I hesitate to try to edit the file manually for confirmation in fear of interrupting the currently running schedule.
[Kup settings]
Backups enabled=true
Number of backups=1
[Plan/1]
Accumulated usage time=2520
Ask first=false
Check backups=true
Destination type=0
Filesystem destination path=file:///datadisk
Generate recovery info=true
Last available space=480422633472
Last backup size=10666175251
Last complete backup=2024,5,10,11,40,39.589
Paths excluded=/home/user/.cache,/home/user/.local/share/Trash,/home/user/.local/share/baloo
Show hidden folders=true
Usage limit=1
Update: After another backup, the quoted line changed to “Accumulated usage time=120”, so this is not where the interval is stored. But the relevant line could be “Usage limit=1”. In that case we can replan by editing this file. I’ll not do the experiment.