I've seen something similar on a Portra 400 roll I shot this summer. There are stains on the edges of the film and they bleed on 2 or 3 frames. On the surface of the film affected by the stains, you can read the text from the backing paper (I'm sorry I can't attach an image yet, nor link to one, as this is my first post). On one on the bad frames, the sky is partially stained ; the stain is lighter than the unstained sky, and the letters from the backing paper have the density of the unstained sky (they're darker than the surrounding stain, as if the film hadn't been stained where it was in contact with the ink). So it looks rather different from the OP picture, where the letters appear lighter.
The film was fresh when I shot it, but the humidity was high (seaside) and the lab might have mishandled the film (it also had scratches, and even traces of glue on the emulsion side...). I've shot a few rolls of T-Max 400 and Portra 400 in the same conditions (the color films being processed by the same lab), and they all came out fine. All in all I'm not sure what caused the issue.