Not sure if this is the issue you're facing, but I always found that User changes wouldn't affect a User until a they log out and log back in (For example role changes)
Am I understanding correctly that you have set the Page URL property (meaning users can directly access the page without deeplink), but you are still using the deeplink module to open the page? If so, are you only doing this to change the language?
I've experienced the issue you are describing where users are redirected to landing page, but that usually happened when the input object for the deeplink returns empty. So it's strange since you have said that the page is not expecting an input object.
My guess is that the URL might be malformed for some reason during the process, although I have no idea why that would happen when using the Arabic language, as apposed to it working when setting to English.
There are multiple previous questions with regards to changing an anonymous user's language, not sure if you've been through them, but if you search for "anonymous change language" you can have a look at those - seems pretty tricky indeed, and you have to take things into consideration like if the client side is aware of the language change.
Have you tried debugging with log level set to debug? Curious to see if there are any errors/warnings like "No session found for deeplink" ??
We have the same issue with deeplink when we use the Arabic language.
The issue is related to the index page, when the Arabic language is used, the index.html page will be index-rtl.html. so you can fix the issue by creating two deeplinks, one for Arabic language and the other one for other languages. make sure to change the deeplink configuration for the Arabic language to set the index page to be (index-rtl.html)