Strange spots in cyanotypes.

Hello everyone, I just finished toning a cyanotype in coffee. The print was previously bleached on a washing soda solution. The print developed countless dense spots after the toning. These were not present after the initial exposure or after bleaching. Are these related to impurities in the paper? Below is a 100% crop of the scanned print.

Thanks in advance

I got something similar once when I didn't filter out the instant coffee grounds.