I think perhaps second. Tests of the RD-250 engine for the Hwasong-12 intermediate-range ballistic missile and the Hwasong-14 and 15 intercontinental ballistic missiles were made up there (Dongchangri) before they might be modifying or polishing liquid fuel capabilities. Don’t have a team in the ground there so speculative first.
Preps for even a liquid fuel tests would create a footprint that would be noticeable before and after. While polishing liquid fuel (reliability) is one possibility I think the push towards solids must be considered due to mobility and quick fire necessary for a potential nuke launch.