When you have a bike that is comfortable to ride outside but leads to problems when riding inside with Trainer, it is important to look at what is different between the use cases.
When we mount a bike onto a typical trainer, it becomes rigidly clamped to it, disallowing any motion. It exerts excessive pressure on the support tissue and discomfort on the saddle. This is because there is no shift in the demand on the muscles and tissue around them.
However, while riding outdoors, we experience natural motion as the bike remains unrestricted in use. Hence, the bike & saddle movement is flexible & dynamic which leads to higher saddle comfort and lower pressure on the sit bones & soft tissue.