How can I delete some of the older messages from conversations but keep others?

How can I delete some of the older messages from conversations but keep others? When I try this by clicking on the arrow on the top right of each message and selecting delete there is the risk that the whole thread gets deleted. Sometimes it works fine, sometimes it doesn’t

Hello Sue,

Your assessment is correct. Deleting one message in a conversation may also delete
the whole converstion because they’re linked.

One way to be safe, is to disable Conversations - even temporarily.
Menu / Tools / Settings / Mail /

or   per message extending to Conversations

Be aware of the settings on the server where you might also have Conversations
set as Default. 

I haven’t tested this  yet, because my default is No Conversatons on both.
the server and the email client…

Pick an email in an unimportant conversation to test if the mail  is affected if the above is the case.

This is not an eM Client probem, but a general mail settings problem
regarding Conversations.

I have the same problem. Pretty annoying as often in longer conversations, you want to only keep specific emails. Also switching off the “conversations” view is not an option. In Apple Mail you also have a “conversation view”. Still, once you delete a message, only that one message is deleted. The remaining thread stays.
Therefore, I would say that’s an eM Client problem.