Why does TLS1.2 break connections to SQL Server?
Older versions of SQL Server and SQL Server clients simply don't support TLS 1.2.  Please check out  TLS 1.2 support for Microsoft SQL Server to see what you might need, or just upgrade SQL Server to the latest service pack/update, and the client to the latest version.  Since I see you're using SQL Server 2017, you should be fine on the server side and you should just need to address this on the client side.  Based on your stack trace, I'm thinking that a .NET upgrade will address this for you.

We have numerous connections using various versions and platforms of SQL clients (including jTDS) connected using TLS 1.2 with various versions of SQL Server.  The latest official release of jTDS doesn't support TLS 1.2, so you have to download an unreleased dev version, or convert it to JDBC, which is really the best option as jTDS is a dead project.