Migrating default contenttypes from Archetypes to Dexterity removes history from existing Dexterity contenttypes

I'm trying to migrate a Plone 4.3 site to Dexterity. The first move is to convert to Plone 4.3 plone.app.contenttypes and later move to Plone 5. The default contenttypes are still Archetypes (Edit: I wrote dexterity here earlier), but some add'ons and custom contenttypes are already Dexterity.

I know the history of the migrated default AT content is not available anymore when it is moved to Dexterity (see my question at Saving old revisions of content when migrating to Plone 5).

But I'm now finding out that the existing Dexterity content, which also has history in the historiesstorage, is also truncated and gone. Which makes no sense because the @@atct_migrator should only touch and migrate Archetypes content and has no business with DX contenttypes already present. Did anyone else notice this? Your existing DX content might not be (history) safe when you migrate at the moment...

I know there have been quite some improvements in the @@atct_migrator if you go 'directly' from 4.3 AT to Plone 5.1 Dexterity with plone.app.contenttypes 1.4. For example to migrated custom/extended fields. I've backported some fixes to plone.ap.contenttypes 1.1.x, but let this be a warning for others who want to migrate using 4.x plone.app.contentypes as an intermediary step. (as was suggested in this migration thread from last year: Problem migrating from Archetypes to Dexterity via @@atct_migrator)