the short answer is: you have probably differently configured logger. They log in a different format or level.
The longer answer: all source connectors currently use the legacy source thread. That will only change with FLIP-27  being widely adapted. It was originally planned to come sooner, that's why the name of the source thread contains "legacy".
Even a bit further into the details: in Flink <1.9, each task used several threads for doing things. With 1.9, all tasks now use a single thread with a mailbox model (kind of like an java.util.concurrent.Executor). However, one type of tasks couldn't be refactored: source tasks. They had to stick with the old model, because the source interfaces assume that each source connector spawns his own thread and pushes it's messages. The new interfaces with FLIP-27 will be pull-based, so that we can also use the mailbox model for that.