Tested on linux ubuntu 20.4 with the latest Bitwig Host. Verified by other users and also with the demo juce plugin.
All our plugins have this problem and it looks like this happens even when no component has the focus. We also set EDITOR_WANTS_KEYBOARD_FOCUS FALSE.
After creating a new VST3 instance and clicking on the plugin UI, the plugin steals the spacebar key and the host does not start/stop the transport anymore.
On macOS it works as expected and Bitwigs start/stop triggers with the spacebar key also when the plugin window is active and clicked.