Using the Rotation: tag on a minecart with a display tile works for only a small amount of time. After being summoned, in about one minute, the minecart will flip back to a default rotation of 0f,0f.
Pushing the minecart will also affect its rotation, which was not true when this tag was first implemented.
Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases. If this has been done, we can reopen the issue.
Keep in mind that the "Resolved"-Status on this ticket just means "Answered", and that we are waiting for further information on whether this issue still exists or not. We will reopen it as soon as the requested information has been delivered.