having saddle discomfort while riding on the trainer, but not outdoors, is really common. Even if you don't realize it, you move around on the saddle much more outdoors. For example, taking pressure off the saddle, even ever so slightly, when hitting a bump/rough road. Also, position on the bike tends to be a little different indoors as compared to on the road. Indoors everyone tends to sit up more as compared to a more stretched out position over the handlebars that is more comfortable/natural outdoors. This more upright position puts more pressure on your sit bones.

Some combination of these 2 factors probably explains the discomfort you are feeling. All I can recommend is to try to move around more in the saddle indoors. Standing is awkward at first on the trainer because the bike doesn't move under you like it does on the road. But speaking from countless hours of indoor experience this past winter, standing will very quickly feel normal inside, and it should help you a lot. Also change your hand positions during your workout. Get into your drops for sprints/short intervals and get well over your hoods for longer intervals. This will also help you move pressure away from your sit bones.