Hi Marco! Thanks for reporting https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1012611 in Debian.
The bug you describe is not due to anything in the Debian packaging, but most likely a common bug for upstream https://github.com/aristocratos/btop. Do you have experience in C++ development? Do you want to take a stab in the open source spirit to fix this issue yourself? This command-line tool is pretty simple, and building and rebuilding it yourself while testing that your changes work is relatively trivial if you have any C background. If you end up submitting a PR upstream, please mark this Debian bug as "Forwarded".