Thank you for your insight,
The result of both Train and Val sets= Training data

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Next, the result of Train = first half of Training data, Val = second half of Training data

rEnlSFDB1600×500 32.9 KB

Note the validation error is very high.
Now would it be correct for me to say, “The problem we face is not due to distribution shift between Training and Validation sets but rather the problem difficulty of learning with generalization”. Because, even when the data was from the same distribution it did not generalize.
Next I am going to try data augmentation (which I believe has been proven to improve generalization as well as to better performance in training). Are there any specific strategies you know that I should follow?