If I add my own statusmessages to add-form, I notice that the 'error messages' are not 'normal statusmessages'. Is there a reason for this?
Basically, I think they should 'render in the same place'?
Since nobody else replied...
This has to do with how validation of your fields takes place. A while ago, I tried to find a uniform approach to statusmessages/errormessages in my own packages (Plone 5). I have not discovered a clean way to do so, but here are some pieces of documentation that clarify why (below is copy paste from my notes). The most straightforward way is probably to add your own add/edit view for your content types.
The approaches below are governed by zope.schema, plone.app.dexterity, z3c.form and zope.interface
Validators documentation: Validators — Plone Documentation v5.2
Validation in schema driven forms: Validation — Plone Documentation v5.2
Plone 5 Training: Dexterity Reference: Dexterity: Reference – Mastering Plone 5 Development — Plone Training 2022 2022 documentation
z3c.form documentation: z3c.form library — Plone Documentation v5.2
Actually, I noticed this WHEN adding my own add-view, so not sure if that helps (but I will give it a try)