Please file an issue with jupyterlab, and reference the corresponding issue jupyter/jupyter_client#259 that was fixed with Jupyter. I’ve filed incorrect cursor_pos for completion requests with non-BMP Unicode characters · Issue #13961 · jupyterlab/jupyterlab · GitHub
Actually, hang on, it might be IJulia’s fault if it is mis-identifying the protocol version here. Let me check it. Nope, it definitely looks like the same old bug in JupyterLab — if I force IJulia to use the old buggy UTF-16 protocol by changing VersionNumber(m.header["version"]) ≥ v"5.2" to false, then it works in JupyterLab (but not in Jupyter).