Joachim, Nitro does not delete previews on its own.
The issue with Hasselblad is that the camera provides very low-resolution previews despite the camera’s high resolution. On the Mac, there are settings that will cause the app to generate high resolution previews automatically. Turn off Quick Preview and/or turn off the setting that makes previews while in Quick Preview. You may need to try combinations of them.
Browsing may slow down when you do that, but the app will build full resolution previews as you view images. You can also use the Build Thumbnails feature to do it in a batch. Most apps avoid this problem by having an explicit Import step, where they will make all of these images before you can work with them. Nitro does not do that – it gives you access immediately. However, for cameras with poor previews, low resolution previews are the downside. But, as I wrote Nitro has features to reduce the issue.
The previews themselves can be large and the app uses a high quality setting when making them, which leads to disk space use. I have considered adding a preference to control the preview quality. Maybe in the future, but I think it causes as many problems as it solves.