Clear conversation (sometimes NOT possible)

Running Jami on Ubuntu Linux 20.04LTS. Always updated correctly.

For some contact it is possible to delete/clear the conversation and for others it is not possible.

How to delete: right click on contact → menu item : Clear conversation

What is the criteria for the option to delete/clear the conversation appears in that menu?
Why is there a criteria to be able to delete/clear a conversation?

Hope someone can help to answer this, because it seems very odd as to why one should not be able to delete/clear a conversation.