In MCreator 2019.5, and previous versions (tested with 1.9.1 as well), the native macOS keybindings appear to be inconsistent with MCreator.
On macOS, keyboard shortcuts such as CTRL+C, CTRL+S, etc are done with CMD, so they'd be CMD+C and CMD+S, etc.
When using the IDE in MCreator, i've noticed that copying works as CMD+C and pasting works as CMD+V, but I've noticed that saving doesn't happen with CMD+S. To save, I have to press CTRL+S. Some shortcuts are CMD+(key) and some are (CTRL)+key.