The <section end="" /> tag does not support a space around an equal sign (=). Example in Russian Wiskisource.
In the HTML specification no prohibition on such spaces. Although personally I think that such spaces are unnecessary. But if some user puts them, this should not cause a silent inoperability of the code, due to which other users are forced to spend time looking for errors. It need to either add support, or write this nuance in the help for extension-LabeledSectionTransclusion and mediawiki-extensions-ProofreadPage in all projects and interwikis. Thanks.