Been a while since I used a Galaxy S7, I don't believe S memos has a trash, you can try a recovery app.
I would guess that if I understand you correctly and it's not the full memo rather just part of a memo that got deleted, it will be very difficult to recover.
After spending too much time with unknowledgable CSRs at Samsung (they were nice, but had to look up what to do with each question of mine) in addition to all my research time, I...figured out what to do by mistake. TYH.
At one point my finger rested too long in one place (ie, the move for "paste") and lo and behold...it's all back!!!
I must have "cut" the highlighted info by mistake to begin with (I wouldn't know how to repeat that) and then I also "pasted" it back by mistake.
The reason I am writing this in detail is to give all of you techies a real life experience of simple solutions to try with an ice pick before jackhammering it.