I've just resumed my PoE playthrough (I had a lot non-gaming issues on my plate lately). On steam, I had to verify the integrity of my game cache, and had to redownload 247 files.
After that, I got the +6 Accuracy boost from my Weapon Focus talent.
Although I have no idea how 247 files could have become corrupted, as I didn't do anything which could have influenced my PoE files in any way, it seems that these corrupted files were the culprit, not the beta patch...
I do sincerely apologize for keeping you busy with this issue unnecessarily.
But thank you for providing us with this marvellous game (and the even better expansions, which made me really look forward to PoE 2)!
Erstarrung's post in [3.03] Stormcaller Bug was marked as the answer