The API described at https://developer.paypal.com/docs/api/transaction-search/v1/ is no longer returning "transaction_note", for transactions that definitely have notes, and which returned notes previously.
Our code hasn't changed and I can also look at the raw json to see that the transaction_note value is simply no longer there, including for older transactions that had their value returned before.
If I load the TXT log of the associated mass pay, I can see the note we provided.
The only time that the value is being returned in the API is when it's a Paypal-assigned note, for example "Mass Pay Reversal".
It was working as of at least March of this year.