User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:120.0) Gecko/20100101 Firefox/120.0
Steps to reproduce:
Using Thunderbird 115.4.1 under Ubuntu 20.04.6.
Viewing list of emails in Inbox, select one by clicking on it, click on Delete button in the Unified Toolbar. The entry disappears as desired. With the next email in the list now highlighted, press Enter key. Instead of viewing this email, it too disappears.
If, instead of using the Delete button in the Unified Toolbar, the Delete key on the keyboard is used, then the repeated deletion from Enter does NOT occur.
The messages that disappeared can be found in the Deleted folder, and restored by dragging back to Inbox.
From a list of emails I expect Enter key always to open the selected email for viewing. This was what happened in previous Thunderbird releases.