SNAP Multi-size Mosaic problem

I am trying to mosaic 3 S2 L1C images. I used the multi-size mosaic to prevent resampling as preprocess for the mosaic-only. However the resulting image has 0 values.

This is the result:

Since S2 images were big, I also tried subsetting the 3 images and repeated the steps above. The result is just the same. How to do this correctly? Should I just use resample then mosaic (not multi-size mosaic)?

Note: I also posted this as question in SNAP forum but it is still for approval.