Apple Mail export to DTPO

Exported all Apple mail (via DTPO plugin) into DTPO. It is visible, but only a small subset of the conversations seems to be in place. Unfortunately I was too enthusiastic with DTPO that I removed all this mail from Apple Mail.

Selected 130 or so email messages that formed a couple of conversations and exported these from DTPO into a Unix mbox. Imported this mbox into Apple Mail and my conversations are back. So far OK.

Created a new database in DTPO. Exported those 130 or so emails back to DTPO and now all my conversations are gone. If I select the mailbox containing these 130+ messages I have one conversation: that mailbox.

My preferences for DTPO Email that are unchecked are
“previously imported will become replicants”, "Download remote images in HTML messages. All the others (including “import complete conversations” are checked.

What can I do keep my email conversations when moving this email into DTPO?

btw my DTPO version is 2.7.4