When you do File-Import into your 23.976 project, you are converting the framerate during the import, which is not what you want.
When you do a File-Import into your 59.94 project, the footage will import at 59.94 and play back at 59.94, so no slowmotion.
Few options.
If you still have the imported clips in the 59.94 project, go into your 23.976 project, choose File-Open Bin, navigate to your 59.94 project, and open the bin. You can use the clips in your 23.976 project. If you add them to the timeline, they will have a Motion Adapter on them. This causes the clips to still play back at their normal realtime speed, but converted to 23.976fps. If you park on a Motion Adapter and open the Motion Effect Editor (under Tools), you can click ‘Promote’ in the Motion Effects Editor, and this allows you to change the playback speed.
OR: you select the clips in the bin, choose Clip-Source Settings, go to the Playback Rate tab, and change the setting from “clips’s frame rate” to “project’s frame rate”.
If you no longer have the clips you imported in the 59.94 project, it would probably be best to alt drag the source clips into a bin in the 23.976 project (or bring them in via File-Open-Source Browser). This links to the source clips rather than importing them. You can then select the clips and choose Clip-Source Settings-Playback Rate, and set the clips to slow motion playback. After that, you can transcode them into native Avid MXF mediafiles.
HTH