The operand immByteMapBase in aarch64 blindly casts the BarrierSet to CardTableModRefBS and checks if the byte_map_base field points into the heap. This sometimes generates the operand for non-CTMRBS GCs too: ->byte_map_base will resolve some random ptr, and if that happens to be inside the heap, it will generate the operand, causing all sorts of weird trouble.
This is low priority because all current GCs are CardTableModRefBS-derived.