It's not supported to export JSON from 4.3 and import it into 4.1. That 4.1 is able to load the JSON data is a coincidence and there may be problems, such as the feet. If you must do that, you need to choose version 4.1 when exporting JSON -- but be warned doing that may lose data:

I did the steps shown, exporting spineboy but with 4.1 selected, imported that into 4.1: the feet were correct and frame 7 showed 7.

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I don't know why you would see it happen differently, so I'm not sure what we can do. We don't go back to older versions and fix bugs. We are happy if we can fix something in 4.3 that would help when bringing data back to 4.1, but if the bug is in 4.1 then there's nothing we can do.