[BUG] `CMS_PLACEHOLDER_CONF` breaks child plugin restrictions

Description

Jasper has a plugin named TextAndMediaPlugin. This plugin can have two child plugins, one TextAndMediaContentPlugin and one TextAndMediaMediaPlugin. The CMS_PLACEHOLDER_CONF is used to restrict a placeholder's plugins to TextAndMediaPlugin and BlockPlugin.

TextAndMediaPlugin can only have a TextAndMediaContentPlugin and a TextAndMediaMediaPlugin, but the BlockPlugin should allow any plugin as a child.

Steps to reproduce

The plugin classes are configured like this:

class TextAndMediaPlugin(CMSPluginBase):
    model = CMSPlugin
    allow_children = True
    child_classes = ["TextAndMediaContentPlugin", "TextAndMediaMediaPlugin"]

class TextAndMediaContentPlugin(CMSPluginBase):
    model = CMSPlugin
    parent_classes = ["TextAndMediaPlugin"]
    allow_children = True
    require_parent = True

class TextAndMediaMediaPlugin(CMSPluginBase):
    model = CMSPlugin
    parent_classes = ["TextAndMediaPlugin"]
    allow_children = True
    require_parent = True

The relevant part of the CMS placeholder configuration is this:

CMS_PLACEHOLDER_CONF = {
    "content": {
        "name": _("Content"),
        "plugins": ["BlockPlugin", "TextAndMediaPlugin"],
    },
}

Expected behaviour

What I want to achieve is that a TextAndMediaPlugin can only have a TextAndMediaContentPlugin and a TextAndMediaMediaPlugin, but the BlockPlugin should allow any plugin as a child.

Actual behaviour

The above setup works in CMS 3.11, but updating to v5 it doesn't work the same. Using the config above, I can add ANY plugin to the TextAndMediaPlugin, while this should be limited to the two child plugins.

Do you want to help fix this issue?

• Yes, I want to help fix this issue and I will join the channel #pr-reviews on the Discord Server to confirm with the community that a PR is welcome.
• No, I only want to report the issue.