The messages "You have a good feeling about this one!" and "You don't know if you have enough skill to reel this one in." have always meant you had a monster on the line. In the past, the only way I could catch grimmonites is when I got the messages "you have a terrible feeling....", "you're positive....", and about 50/50 when I'd get the message "you're fairly sure....".
I have never ever caught a grimmonite when I got the fishing message shown in the above 2 images before. However, I've been in SSG for about an hour now and I haven't seen one of the 3 reliable messages even once, so I decided to reel in one of the sure-fire monster messages. To my surprise, it was a grimmonite.
When exactly was this changed? Is it a bug, or something intentional from S/E and I somehow missed the memo?