I have copy pasted your exact commands, and that is the output I get. Not sure how to check its reading the exact same file. Here is the output of zpool iostat -v 1 while its reading with your command:
zpool iostat -v 1
capacity operations bandwidth
pool alloc free read write read write
-------------------------------------- ----- ----- ----- ----- ----- -----
boot-pool 1.25G 215G 5 3 80.9K 24.5K
ada2p2 1.25G 215G 5 3 80.9K 24.5K
-------------------------------------- ----- ----- ----- ----- ----- -----
nas_data1 50.1G 36.3T 0 183 678 111M
mirror-0 25.8G 18.2T 0 91 337 57.3M
gptid/561f15f1-826f-11ef-a9e2-04d9f5f3d8ce - - 0 47 16 7 28.6M
gptid/560ccc0a-826f-11ef-a9e2-04d9f5f3d8ce - - 0 44 17 0 28.6M
mirror-1 24.3G 18.2T 0 91 340 54.2M
gptid/5617c9a6-826f-11ef-a9e2-04d9f5f3d8ce - - 0 45 16 7 27.1M
gptid/56260e35-826f-11ef-a9e2-04d9f5f3d8ce - - 0 46 17
This time got even higher read speed, BTW:
zfs set primarycache=all nas_data1
fio --name TESTSeqWrite --eta-newline=5s --filename=fio-tempfile-WSeqARC-OFF.dat --rw=read --bs=1M --size=50G --numjobs=1 --time_based --runtime=60 --group_reporting
TESTSeqWrite: (g=0): rw=read, bs=(R) 1024KiB-1024KiB, (W) 1024KiB-1024KiB, (T) 1024KiB-1024KiB, ioengine=psync, iodepth=1
READ: bw=9292MiB/s (9743MB/s), 9292MiB/s-9292MiB/s (9743MB/s-9743MB/s), io=544GiB (585GB), run=60001-60001msec