On another computer I experienced in synaptic, that the new kernel version 5.10.0-14 is not displayed using apt-xapian-index, but could be found and installed using a search.

Calling update-apt-xapian-index as root from terminal shows some syntax errors that I can not reproduce on the computer I am using now which is running with a similar Devuan configuration. It was something related to python.

But maybe someone here knows the issue and has a solution.