Since you’re printing white first, it’s not likely the strings are from the black of that print.
Could they be something left on the build plate?
What plate are you using and do you clean between prints?
What I have done in the past to ensure I am getting a clean white first layer. Along with a thorough cleaning of the physical plate is add a new plate in Bambu studio, add a primitive cube to that plate set to 50mm x 50mm x .2mm and print that, verify that it’s all white, if not I print again. after a clean print I proceed to print the model making sure white is the first color to print.
I preheat my nozzle before the print and wipe anything left on it while heat up to get dark colored residue off from aound the nozzle tip when I plan on printing w white and need it to be clean. That is most of the time the culprit of your problem
Theoretically it should be fine w just wiping the tip clean. I get black blobs like these on light colored or even transparent prints like this even after 4-5 prints between black and these w residue on them. The problem imo isn’t that there is leftover filament inside the hotend but between the silicone and the nozzle tip. Attached pic of said black (pla) residue on a petg vase print, which was printed 4 models after the last usage of black pla. Nozzle wiper is useless, I’m waiting on my a1 wiper silicone to come to mod my p1s wiper, they are much better (I heard)
What I was suggesting was a large flat square .2mm high should be one layer high, if you cover the area of the build plate you will be printing your model on it might detect any contamination of the build plate that you could not see after cleaning. I am not sure what is causing your issue, I have printed several multi color logos without that type of contamination. Maybe try a different brand black filament.