That’s exactly story of my plants
Baby care then and next session they are dead , lots lot of plant
Last year have 7-8 mango trees from Florida , and 3 Manila seedling , overwinter in garage
Out of that only 1 Florida mango is relatively okay (Maha Chanok ) and 2 are barely surviving
All 3 Manila seedling was in top notch condition, planted 2 in ground already this season
I believe it has to to dump conditions of the soil during the winter times, apparently Florida root stock is not good at it
Simon , did you try to bareroot the florida rootstocks tree and planted in ground , this is just eliminates the possibility of soil around the rootball causing the root rot
Keep us updated with your experiment, probably start doing the grafting for people like us