Does the account use POP, or IMAP?

If IMAP, your Trash folder is almost certainly on the server and as such, may be subject to the email provider's policy on trash/deleted folders.

Check what your provider's rules are on trash retention. You may be permitted to adjust the period for which they are held. Alternatively you can set Thunderbird to use a trash folder under your Local Folders account which will be on your own hard disk and thus invisible to your email provider.

And of course check the settings in Thunderbird for what to do with deleted messages. You can set it to automatically remove messages after a defined period. Look under Tools|Account Settings for deletion settings.

OTOH, if you're routinely deleting messages and relying on them being kept indefinitely, I'd suggest you rethink your procedure and not delete them until you're confident you no longer need them. Recycle bins, waste bins and the like are the first to go when disk space gets tight. Most email providers have a policy of automatically removing messages over a certain age from the Deleted folder, specifically to manage and reduce disk space usage.

Thanks for your explanation!