Hello Martin, thanks for your question.
I believe it is the functionality that is in the actual extension not the core product. There may have been some API changes in how functionality such as this was enabled.
Below is the D3 charts extension Qlik Branch - notice I DO have the Convert to with this extension.
Check out some other extensions and let me know.
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Thank you for your reply and as always your input is greatly appreciated.
As in my original thread converting from an extension to a native Qlik Sense object works.
Converting an object to an extension object is what is not working anymore and has regressed in 3.0.
E.g. try to converting your Bar Chart back to the D3 charts extension in your example...