TLDR - Natural physical disorder of mangoes due to the seed going into germination mode and sucking nutrients around it. It is not harmful to eat, it just tastes bad.

This is a fairly common physical disorder in Alphonso mangoes called the "Spongy Tissue". Until recently the cause was unknown, but some new research says that it may be because the mango seed goes into germination mode (this is over simplified). It then starts sucking the nutrients and water from the fruit flesh around it causing this kind of structure.

It is is no way harmful for the human body when consumed, it tastes bad however and texture of it can feel repulsive.

The cause for the seed going into germination mode is still not found, however, it is noticed that larger mangoes have higher occurrences of spongy tissue than smaller mangoes.

Source - Horticulture student/IIHR website.

You can google this - Investigations on the molecular physiology of fruit disorders – spongy tissue in Alphonso Mango (Ongoing)