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Check the proj definition stored in the spatial_ref_sys, it should be different from the one used in the proj4js definition. For example, mine is the following: +proj=utm +zone=30 +ellps=intl +towgs84=-87,-98,-121,0,0,0,0 +units=m +no_defs Also epsg.io gives the one with the towgs84 parameters, so I'd change the Proj4js def.
This aspect of plugin development is indeed poorly documented. I have run into the same problem and already posted a few answers to related questions : QGIS - Edit UI - Python Init function localization with QTranslator Multi Language support for QGIS python plugin Hope this will help you.
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