I tried the same and it works on release 2.2.0 alpha.
I opened
both files together in the same LibreCAD session.
Some entities from drawing A are copied with insertion point and pasted into drawing B. Layer 3 is new and the line thickness is still 0.5 mm. For the others layers, the entities are pasted and the line thickness is still kept.
Let me know if it works for you.
Fabrice
French hobbyist interested in 2D design.