Hi, This is the default behavior of Aqua Mail. By default, the "Select All" function only selects the messages visible on the screen, not all of them in the folder. This is because the app loads messages gradually to save memory—scrolling loads more.
Change settings:
Go to Settings > Mail list view (or "Message list view"). There you can change "Select All" from "visible messages" to "loaded messages" to select more (loaded) messages.
To load more of all emails or synchronize the folder completely.
"Loaded" depends on the cache: Settings > Accounts > Folders & Options > Increase message caching (e.g., 1000).
After editing: Refresh the folder and scroll down to the end to load more