I tried an accelerando over hundreds of bars. It looked the same in Galley View and Page View. It allowed me to extend it as far as I liked, except it would not go past a tempo indication.
As is usually suggested, it will be much easier for others to diagnose if you post the project here, or at least the section where the problem occurs. If it does turn out out to be some sort of unintended behaviour, glitch/bug, etc., the Dorico dev. team will be able to fix it quicker if they have an actual example of what is causing it.
If you select the “5” without selecting anything else, you can delete it and all is as it should be. Before you delete it, open the Properties panel - you will see that it is Tempo text. As mentioned in my previous post, an accel. mark will not extend past a Tempo marking.