Development tools for Zephyr on STM32Workbench for Zephyr: VS Code extension to set up, build, flash and debug Zephyr applications on STM32.Devicetree manager: browse, edit and visualize Zephyr devicetree, STM32CubeMX-like for Zephyr.
What happens if you send the working character over and over again? Does it flake out sometimes? If so it could be a signal strength problem. I'd scope it and look at the data if you can.