A damp tank, or a trace of residual chemicals would be my interpretation. I have seen that sort of mark/colouration with roll film where the end of the roll has been in contact with the film further in. You might be able to clear the marks in fresh fixer - it would do no harm to try.
I do 8x10 in Jobo drums, but I only load the (dry) drum when I am ready to process. The reason for this is that the sheet film is close to flat normally, and that is enough to hold it in pace against the tank ribs. If the film develops a curl there may not be enough tension.