Hello,
To connect SQL Server with instance name from SnapDevelop 2703
1. First we need to connect SQL Server without instance name (eg. computername instead of computername\instancename)
How to do it ?
a. In the SQL Server configuration Manager, under SQL Server Network Configuration -> Protocols for SQL2012 -> Right Click TCP/IP -> Change TCP port to 1433 for IPAll

2. Open port in Firewall for 1433 port
3. Restart SQL Server, if required.
4. Try connecting SQL Server from SQL Management studio with computername alone, not with instance name (eg. mycomputer instead of mycomputer\instancename)
5. Now try connecting from SnapDevelop (with computername alone).
6. See no other instances of SQL Server running in that machine.
It's working for me.
You mean, 2019 R3 LTS version won't have this bug fixed ?