The specific dye gives maximum absorbance at the lambda max provided by the supplier/labelled on dye under dilute aqueous phase. However when the same dye solution at high concentration when operated for spectral scan, deviates.
Self-association of dye molecules at high concentrations could alter the absorbance spectrum. Excessive absorbance that goes beyond the dynamic range of the spectrophotometer (e.g. absorbance >3) would cause flattening of the spectrum.
Guobing Xiang probably identified the main problem: the Lambert-Beer law is valid only over a limited range of absorbance. Another problem can be that "analytical grade" in dyes means little, there may be significant contaminants. Do a thin-layer chromatography (Kieselgel, butanol/acetic acid/water) with analytical grade eosin and you'll know what I mean. Yuck!