This sounds like something specific to you. I did struggle to find an account with trash that I could server as I don't keep it. but I eventually found a couple of emails in a POP account and some others in local folder. I right clicked the folder, selected search and searched on body information from the email and got matches.

Have you tried restarting Thunderbird in troubleshooting mode from the help menu and continuing when prompted to see if you can search in that mode?