there is a plugin called "BossShopPro" and it has a feature about MiddleMouseButtom to create an action for selling or buying.
it works on 1.17 with the MiddleMouseButton action but not in 1.18

this might be cause of BossShopPro, but just added in here just in case if it was ProtocolLib.
if its not don't worry about writing back, big love from Achy, love your work!