Be aware of the fact tiering with ReFS + Windows Server 2019 works in a totally different way compared to NTFS + Windows Server 2016.
2016: All writes go to the flash tier with the data being offloaded to spinning disk when flash is getting full / close to being full (~80% usable capacity occupied AFAIR).
2019: All writes go immediately to the spinning disk and data get "escalated" to the flash tier only if it's getting accessed frequently (using temperature map).
This means if you're using DiskSPD or FIO or other benchmark with WS2019 you'll be seeing disk-only performance initially. Give it some time and pinpoint smaller workload to flash to see the real performance.
P.S. It's VERY confusing! Microsoft had broken lots of apps / setups doing things like that...