Placing Items in the off-hand that can't be placed in the off-hand results in you dropping the item. (Regularly, it just prevents you from placing it into your offhand, similar to how you can't place a non-armor item in your armor slots)

Unsure if this is intended or no, but torches cannot be placed into the off-hand either, which is a parity issue between Java and Bedrock (and also reallllllly annoying)