The designer isn't using a direct reference, it loads the assembly through the DbProviderFactory system in .NET. This system reads from the machine.config file the factory associated with the name "Devart.Data.MySql".
There are a couple of machine.config files on a system, one for 32bit and one for 64bit. Unless you explicitly start the 32bit runner, you start the designer in 64bit and therefore have to look in the machine.config file: c:\windows\Microsoft.NET\Framework64\config\machine.config
It should have a line like:
<add name="dotConnect for MySQL" invariant="Devart.Data.MySql"
description="Devart dotConnect for MySQL" type="Devart.Data.MySql.MySqlProviderFactory, Devart.Data.MySql, Version=8.12.1229.0, Culture=neutral, PublicKeyToken=09af7300eec23701" />
So either you're running the designer by accident as 32bit (and therefore use an outdated machine.config file) or the machine.config file isn't updated by the installer of devart.
But in any case: our code doesn't have any reference with an ado.net provider other than sqlclient: they're all loaded through DbProviderFactory.