Yes, the Canon EOS R6 Mark II is compatible with the Samsung PRO Plus 256GB SD.
Samsung PRO Plus 256GB SD is V30 class. Meets the floor for: 4K30 (30 MB/s floor). Does not meet the floor for: 4K60 (60 MB/s floor). (Peak write 90 MB/s is not evidence of sustained performance; the V30 class rating above decides this.)
Limitation
Cannot record 4K60 in 10-bit or Canon Log 3 -- requires Video Speed Class V60; this card is V30 class. Works for 4K60 8-bit (requires UHS Speed Class 3, which V30 meets) and all 4K30 modes. Upgrade to a V60-rated card for full 10-bit/Log capability.
“Card performance requirements (movie recording) [write/read speed] Movie recording size 8 bits 10 bits (Canon Log 3 / HDR PQ) 4K UHD 4K UHD cropped 59.94 fps 50.00 fps IPB (Standard) UHS Speed Class 3 or higher Video Speed Class V60 or higher IPB (Light) UHS Speed Class 3 or higher”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)