Rolf,
I think it may help to read-up on dpkg and apt. Instead of 'whereis' do
dpkg -l r-base-core
to see the content of that package. Use
apt-cache policy r-base-core
to see what the package cache says about where it gets the package from. And
so on. Also
type -p R
which R
will tell where your system thinks R comes from. If you install R 3.5.2 you
should not see R 3.5.1.
These aren't really R issues. The package management system can be pure
source of joy. Something seems to have gotten crossed at your end but we
cannot tell what.
Dirk