Modifications of profiles, or at least what I experience it with, the JVM arguments, do not save when you apply them, thus upon game crash within the session of modification, the changes will revert.

How to reproduce:

1. Go to the Minecraft Launcher
2. Find a profile in the list of profiles
3. Expand the options and choose Edit
4. Click More Options to expand
5. Change an argument like -Xmx or -Xms to a different value
6. "Save" the changes
7. Launch the game with the modified profile
8. Cause the game to crash (It may be certain crashed but I have experienced them on various)
9. Go to the Minecraft Launcher again and go to edit the profile
10. Find the argument you modified and notice that it has reverted to previous value