In Jain tradition, Chandanbala’s offering was linked with Bhagwan Mahavir’s parna after his six-month fast, not with the moment of kevalgyaan.

Bhagwan Mahavir attained kevalgyaan later, at Jrimbhikgram, under the Shal tree, on the bank of the Rijubalika river.

So the sequence is:

1. Chandanbala offered ahar to Bhagwan Mahavir
2. His six-month fast was completed
3. Later, Bhagwan Mahavir attained kevalgyaan

If you want the full Jain story of Chandanbala and Mahavir Swami in simple language, I can explain it.