Hi Kiran ,
Not sure but it might be the standard behavior where the sales agreement transaction type is not displayed in the transaction history AB for the reference document assigned for opportunity .
Also kindly check for the object relationship profile for opportunity ,you can see that campaign will be maintained as the reference object in the profile hence in the screenshot posted above campaign can be seen in the transaction history as reference document link .
, where as sales agreement will not be maintained in the profile .
Check for the SPRO path :
Transaction->Basic settings ->Setting for opportunity ->Define object relationship profile .
Kindly go through the defined documentation for transaction history as below :
Hope this might help .
Regards
Shweta