Hi @Stella28,
The issue of negative eccentricity with the BrowerLyddane Propagator has been corrected in Orekit release 12.2.
Problems converting osculating elements into mean elements can affect propagators based on mean theories (Brouwer-Lyddane, Eckstein-Hechler, DSST, SGP4/SDP4 for TLEs). Upcoming release 13.0 (coming soon) will enable selection of the conversion algorithm (fixed point or least squares) for improved conversion control.