After changes in MaxMind licensing model and before upgrading to Matomo v3.13.1, I decided to become a register user of MaxMind in order to continue using their geo database.

Then, after I finished to upgrade my Matomo instance to v3.13.1, taking into account the recomendations from Matomo, I decided to give a chance to the new provider db-ip.com.
The experience was very gratefull because the "Unkown city" metrics fell down drastically as shown in the following evolution graph:
(the vertical line indicates the day of the upgrade to v3.13.1)

However, the drawback is that "Unknown region" has raised, also dramatically:

I was warned about that Geo-IP free databases aren't 100% accurated. However, given the huge difference between the two providers, I'm wondering if it could be a kind of v3.13 bug.
(I reviewed my installation several times but I'm not able to see any clue about the origin of this effect.)

Any help will be wellcome!