The above is most likely caused by the Sampling Point Size to Padding Ratio of your font asset being too small combined with user material property values near their max range.
When material property values are near their max range, you are at the edge of the adjacent glyph in the atlas texture where as a result of bilinear filtering we can get these artifacts.
The solution is to increase this Sampling Point Size to Padding Ratio where for instance, if your sampling point size is 80 and padding is 8 for a ratio of 10%, then increase the padding to 12 will increase the effective range of material properties where you will be able to use smaller values to achieve the same visual FX but without the visual artifacts.
See FAQ Question #2 and #10 from this thread for more information.