Without any error message or warning? No! You should at least get the "There is not enough disk space to download new messages" alert. What lets you think that Thunderbird deletes your stored emails?

Thank you for your reply. I can see that the deletion of my emails is actually being carried out by Thunderbird itself by using its Activity Manager (Tools → Activity Manager). But no relevant warning messages have appeared, apart from a notice stating that compacting the accounts could not be performed due to insufficient storage space. Kind regards.

How much free space do you have on your hard disk? Which is your retention policy in Account Settings > Synchronisation & Storage > Disk Space? And which is your setting in Inbox > Properties > Retention Policy?

My Thunderbird profile folder is located in a small disk partition (8 GB) that is almost full (150 MB free). The Thunderbird application itself is located on the system partition, where there is still enough free space. Retention policy (in Account Settings > Synchronization & Storage > Disk Space and in Inbox > Properties): ‘Don’t delete any messages’ and ‘Always keep starred messages.’ Thank you.

For which reason did you move your Thunderbird profile folder in a disk partition which becomes too limited to store new messages? But even if there is not enough disk space I can't believe that Thunderbird automatically deletes older messages without displaying a warning, unless you had yourself set up a limited conservation delay. Did you review retention policy in Account Settings > account name > Synchronisation & Storage > Disk Space? Did you review retention policy of your Inbox folder > Properties > Retention Policy?