Sotapanna new jati/bhava

“I was wondering suppose a human reaches Sotapanna stage and dies to be born as a human again or in some other bhava, how does he/she remember the Dhamma that was learned from the past life?”

The same way that a child remembers a past life.
– The only difference is that a Sotapanna’s mind will never lose that “knowledge” whether born as a human or deva.

“Let’s assume a sotapanna dies without grasping paticca samauppada..”

It is unlikely that one would attain the Sotapanna stage without grasping Paticca Samuppada (PS).
– Now, that does not mean one needs to know the formal PS, i.e., “avijja paccaya sankhara”, ..steps one by one.
– One would need to know that suffering arises because one generates defiled thoughts based on one’s ignorance of the true nature of this world.

This is also related to your first question. One may have investigated this in previous lives and may have made a lot of progress.
– Then one would just need a “little push” to make things clear and to attain not only Sotapanna but even higher stages of magga phala.
– There were many such occurrences, especially in the days of the Buddha. Many people were able to attain magga phala with just a “little push.” For example, Venerables Sariputta, Moggallana, Angulimala, etc.

It is a good idea to contemplate on this important point.