Finetuning T5-small delivers incorrect outputs after finetuning

I am creating a Question answering system using t5-small. I trained it on a Q&A dataset with 300 rows and after training the model showed good performance on inference and outputted perfect answers. But as I finetuned the same trained model on new data the outputs were incorrect.

I tried This with Questioning the old trained model and the new finetuned model with same question but the trained model answered properly and not the finetuned one.

Were you using extractive approach?