Ok, seems like this issue is specific to the single quotes around julia.supportedLanguageIds.
{
"key": "ctrl+enter",
"command": "-language-julia.executeCodeBlockOrSelection",
"when": "editorTextFocus && activeEditor != 'workbench.editor.notebook' && editorLangId in julia.supportedLanguageIds"
}
works fine for me.
Upstream issue:
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