Mid-game addfile stops saving the save slot

When using addfile to load an add-on mid-game, the current save slot will no longer be saved, even if the add-on doesn't "modify" the game. isgamemodified still outputs "modifiedgame is false, you can unlock extras", and Emblems, extras, and other "global" data stay collected/unlocked/intact.
Returning to the title screen and re-loading the save slot will make it save correctly again.

Steps to reproduce:
- Place this file in your preferred add-on folder: SmoothShaders.pk3
- Launch SRB2 without add-ons
- Start a new or load an existing save slot in single player
- Use addfile SmoothShaders.pk3 to load the add-on mid-game
- Observe that isgamemodified says "modifiedgame is false"
- Complete a level
- Return to the title screen

The save slot remains how it was before you loaded it (e.g. GFZ1 with 3 lives, 0 points, and no Chaos Emeralds), not how it was when you quit (e.g. THZ2 with 10 lives, 50,000 points, and three Chaos Emeralds), but any Emblems that you collected remain collected.