What is the CAS latency of the memory in a Mac Mini 2018?
Based on iFixit teardown which says these RAM pcs are Hynix model "HMA851S6CJR6N-VK" I'm assuming that they're the same as the ones you can find by browsing this model name (i.e. here) - and they're described as CL 19. 

However, there's a video of this guy that switches his Mac Mini's RAM to CORSAIR "CMSX32GX4M2A2666C18" that has CL 18 and it seems to work fine.