Do not expect stats to go into the 20-30 range. The hard cap on stats for player characters, whether level 1 or level 20, without certain very specific, unique magic items, for player characters, is 20. At very, very most, certain classes can get some of their physical stats up to 24 at level 20, but there exist no such perks for casting stats.

End game has so far been stated to be around level 10, and 5e rules only define progression up to level 20.

Further to this, as you level up, you will have more levels of spells, and you prepare from what you know. In an un-modded game, you are never going to be asked to prepare more spells than you know. At level one, a wizard knows a minimum of 6 spells, and they can be asked to prepare an absolute maximum of 6 spells. At level 2, they know an absolute minimum of 8 spells, and can be asked to prepare an absolute maximum of 7. At level 3, they know an absolute minimum of 10 spells, and can be asked to prepare an absolute maximum of 8. At level 4, they know an absolute minimum of 12 spells, and can be asked to prepare an absolute maximum of 9. Even if they fine a legendary unique magic item at level 5 and raise their Intelligence to 22, at level five (extremely unlikely), they know an absolute minimum of 14 spells, and can be asked to prepare an absolute maximum of 11.

There are currently 19 spells of 1st level and 12 spells of 2nd level implemented into the game for Wizards at the moment, so no, this problem will not emerge in unmodded games.

You mentioned that you have a screenshot - could you show us one that shows the stats you're working with, on the level up screen, and how many spells it's asking you to prepare?

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That said, I do agree that requiring you to prepare your spells when you level up is not necessary and adds unneeded bloat to the level up screen that they could be using to give us actually valuable information about the game and what we're doing. I would support not being required to prepare your spells at level up, but because it is unnecessary, unhelpful and wasteful of resources... not because it is an issue or has bugs - it doesn't. If they must leave it in, I would definitely support them fixing it up so that preparing the maximum amount that you can is not required, since you aren't ever required to have your maximum number of spells prepared at any other point.