Hi @newWL
I think you encountered the following issue: From Osculating to Mean Elements with Brouwer-Lyddane Propagator, negative eccentricity (#947) · Issues · Orekit / Orekit · GitLab
It is related to the following forum thread: From Osculating to Mean Elements with Brouwer-Lyddane Propagator
What you could try is using EcksteinHechlerPropagator instead of BrouwerLyddanePropagator. ECK models zonal harmonics from J2 to J6 while BL from J2 to J5. Just note that ECK is limited in terms of applicability as described in the documentation: src/main/java/org/orekit/propagation/analytical/EcksteinHechlerPropagator.java · develop · Orekit / Orekit · GitLab
If you have an account on the Gitlab forge, could you 
the issue. Like that we know that several users have the same issue and we can increase its priority.
Best regards,
Bryan