PS: Dear Blues, Is it possible to get this stickied here or in the Bugs forum? Hoping it will reduce the number of threads inquiring about this supposed bug.
For everyone who is experiencing this annoying bug, I've summarized everything we know about it here. Hope it helps.
Brutal 5/10/15/20/25 not credited.
1) Brutal 5/10/15/20/25 is only credited through missions that have no timestamps.
2) The amount of credit is based on the number of missions completed on Brutal in your archive.
1) Start new campaign, Play everything on Brutal
2) Load an old save with at least 1 mission incomplete*. Re-play the completed missions on Brutal, then play the incomplete missions. Similar to option 1), except you have the freedom to choose your mission sequence.
3) Acquire a save file with 24/26 Brutal missions completed (from anyone who is kind enough to share it), Complete 25/26. (Tried & Tested, Works)
Note: A Mission is 'incomplete' if it has no timestamps on it.
Removed all the fluff. Here's the solution in a nutshell.
If there's any doubts feel free to post a question. Thanks to everyone who contributed feedback on the matter to help verify these workarounds.