The epoch version appears in the version string before the real version number and apt considers it higher than any version number without an epoch version, which means that the current MariaDB version 1:10.1.29-6 in the Ubuntu Bionic repo is considered higher than MariaDB 10.2.14 from the MariaDB repo (or the 10.2.15 packages in buildbot). So apt always chooses it.

This has already been fixed in 10.3 by otto in this commit.

From the commit message:

A non-MariaDB.org contributor introduced the epoch 1: in Debian.

There is no way to revert it, so we just need to adapt to it to

ensure no 10.3 installation would downgrade to that version in

Debian/Ubuntu, as 1: is considered higher than any version before.

This fix should be backported to 10.1 and 10.2 so that installs of those versions of MariaDB from the MariaDB repositories will work on Ubuntu Bionic and future versions of Debian and Ubuntu.