Is it soft like a sponge or like soft like pressing on the underside of a plastic Tupperware? Spongey is not good but mailable like tough plastic is normal in hatchlings.
All babies have flexible plastrons. It is what allows them to uncurl from having been in an egg. It should harden up over the first year.
If the shell goes spongey that’s Metabolic Bone Disease (MBD) which is caused by a lack of UVB light which is necessary for the tortoise to produce its own Vitamin D which in turn enables it to process the dietary calcium necessary for a healthy shell and bones.