The title's the best way I can summarize it in one sentence, but to explain more on what I think happened here:

This happened on a border between old and new chunks, and the water area next to this ravine is very deep which I believe is due to the bottom of the ravine influencing the blending, it does not seem have any features you'd find in a River biome like Clay patches, if someone else can come up with a better way on how this happened that'd be nice!

So I tried to replicate this using another seed (-6948714410367329106) and loaded it in 1.17, then I removed the chunks west of a particular ravine (at -363 72 -26).
Then I made a back-up of this and loaded it in 1.18 Pre-release 1 and it's mostly the same result, except the bottom half of the water blocks do update (start flowing out)

The file attached is this world I used to replicate (It's still in the 1.17 format but already has chunks west of the ravine deleted so you can experiment how Blending affects it!)