joint SetPosition() does not work with pysics enabled

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I have a model with two links which are connected with a joint (revolute). At the beginning I have an lower and upper limit, lets say both are zero. I have a model plugin in which I can set the new angle with SetPosition() (upper and lower limit are changed as well). If I run this plugin with physics enabled, the position changes but snaps back to its original position immediately. If I run the plugin with physics disabled, it works just fine.

Any ideas why this is happening?

Thanks