Developer Console suddenly truncating System.Debug output

Sounds like they added something to the Dev console to truncate long lines in spring 19 (On March 20, 2019 according to the Release Note changes).

Developer Console: String Values in Debug Logs Are Truncated

To help prevent connection pool timeouts caused by exceptionally large queries, strings are now truncated at 512 characters in the Developer Console’s Log Inspector. This truncation applies to all string values in the Execution Log panel and to SOQL queries in the Source panel. You can view the complete string in a raw log file.

If you view the raw log you will still see the full log.

This also came up recently in system debug messages get trimmed. I suggested an alternative way to view the debug logs in that answer.