There has been some inaccuracies of the SFT caused by the differences in the basicity of the nitrogen in different amino acids which were explained by S. L. Jodidi. For instances, proline(an amino acid), histidine, and lysine yields too low values compared to the theory. Unlike alpha, monobasic (containing one amino group per molecule) amino acids, these amino (or imino) acids' nitrogens have inconstant basicity, which results in partial reaction with formaldehyde.
In case of tyrosine, the actual results are too high due to the negative hydroxyl group (-OH), which acts as a base. This explanation is supported by the fact that phenylalanine can be accurately titrated.