Debian 9 has MariaDB locked down to 10.1, plus security patches from 10.2 and later. But anything looking still thinks it's 10.1, which for things like NextCloud is a pain. They haven't quite made it outright rejecting such an old DB, but there are quite a lot of warnings about it.
Thankfully, there are repos available for pretty much every distribution to be able to install even up to the latest stable branch directly from the MariaDB Foundation.
Make sure you Add signing key to apt without mass deps.
