Before I was storing it in mysql as float, which (according to the doc on mysql):
FLOAT[(M[,D])] [ZEROFILL] stores floating point numbers in the range of -3.402823466E+38 to -1.175494351E-38 and 1.175494351E-38 to 3.402823466E+38. If precision isn’t specified, or <= 24, it’s SINGLE precision, otherwise FLOAT is DOUBLE precision. When specified alone, precision can range from 0 to 53. If the scale is defined, too, precision may be up to 255, scale up to 253.
Because of comparison issues, I now store the values as strings so mysql never touches them. Maybe your mysql cuts basic floating points at 4 decimals (btw: I have older locations with 5 decimals). Using strings this is a thing of the past and all coordinates are kept as they come in.
Maybe that has the effect you’re seeing (that change was done in eme version 2.0.25 (from 26 March).