In Edit mode in Experience Editor, some of placeholders are not visible after being added. In cases where the SXA module is used, some of them are shown as fallback, even though the Editable setting is enabled on the placeholder item.
The reason for this behavior is because under the hood Sitecore CMS/XP takes placeholders from the cache. This cache is preinitialized on the first use by the following Sitecore query:
sitecore/layout/Placeholder Settings//*[@@templateid = '{5C547D4E-7111-4995-95B0-6B561751BF2E}']
The item is not added to the cache by default because Sitecore CMS/XP limits the maximum number of items in a query result set (considering performance issues). So, in cases where the number of placeholder settings exceeds the one set by the Query.MaxItems setting, extra items are not in the cache.
When changing the placeholder key directly in Content Editor, the issue does not occur, because the cache is updated with the required values.
Consider increasing the Query.MaxItems setting value to 300 or 400 as follows:
<setting name="Query.MaxItems" value="100" />
is replaced with
<setting name="Query.MaxItems" value="300" />
Note: the default value of the setting can be overridden to 260 by Sitecore.ExperienceExplorer.config file for some Sitecore CMS/XP versions. Therefore, check the configuration settings to apply the changes to the proper config file.
You can consider further increases after adding more placeholder settings.