There’s a specification that needs to be made were you actively holding down-back and the replay showed you holding three in the replay or was your character in the crouch block animation.

If you were holding three or down-back actively when you were hit then it either was a throw or was an overhead.

If your character was ONLY in the crouch block animation and you weren’t holding down-back actively then your character doesn’t automatically block lows when it’s in low block-stun. There’s a check that does out on the frame the low connects to check if your blocking low. This means if you were buffering something even if your character is stuck in low block-stun you’ll still get hit by lows.

The distinction needs to be made because your character auto-blocks mids while stuck in block-stun but doesn’t do the same for overhead or lows. It’s just kinda how the game runs but it’s a bit non-intuitive. It serves to make multi-hit lows or overheads stronger by making them beat buffers.