That is a bug, but maybe not the one you expect. The trial is always the very latest version of Spine (beta if a beta is the latest). The trial should not show the settings for changing the version.
Note you are looking at the spine-js branch. If you look at master you'll see that spine-js has been replaced by spine-ts, which is much more advanced and works with exports from 3.5.xx. There is also a 3.6-beta which works with exports from 3.6.xx, but some functionality may be a work in progress.
