This is only a visual bug. The replay still turns out the exact same.
You can take any player's items by spectating them, opening their inventory, and clicking on an item. From my testing if you put an item in the offhand it promptly disappears.

Is Replay Mod the only mod you use? Replay Mod should not allow you to open any inventories or containers nor render any GUI.

Is Replay Mod the only mod you use? Replay Mod should not allow you to open any inventories or containers nor render any GUI.

Just using Fabric 1.21.3 (latest version as of now) and Replay mod (also latest)
It lets me open a players inventory when spectating them, other GUIs don't work.