Glad to know it's not just me!
Related to this, I notice that the error code that is being returned is 1. (ie add "echo $?" to the example) This doesn't seem to be the correct number. However, I see in Wrong Exit code that the issue of incorrect error codes has been brought up before. Has this particular error's code (perhaps it should be 6?) been fixed in the latest version of rclone?
Also, FWIW, I just figured out a way around this issue for my pipeline by first validating json with jq:
rclone blah (json sent to std err) 2>&1 | jq -c 2> /dev/null | (remainder of pipeline)
First I convert rclone json std error to std output and then pipe the json through jq. jq uses std output and the bad line goes to std error which I can then divert to /dev/null