Dixie 2+ lin fm - input level exceeded?

i have a question about the lin fm circuit of the dixie 2+:
when doing lin fm than (depending on the modulator level) i get a distorted kind of freq mod. well, i dont mean the characteristic sound of a high modulation-index … when increasing the attenuator to a certain threshold it seems that the cicuit overshoots and apruptly changes from the nice fm sound to distortion.
it doesnt occur when doing exp fm. so i just wonder if this is intentional and hence part of the lin fm circuit? thanks