Funnily enough, I watched a video of John Leckie who produced the first album and he said Brown never had any problems singing in tune in the studio. Got to remember these were the days before auto tune and pitch correction software like Melodyne. There's a video of them recording the song Elizabeth My Dear in the studio and he is singing it in tune.
It seems he definitely has a problem on stage, maybe he struggled with stage volume when trying to find the right pitch. It can happen if there's poor monitoring, which it often was in those days.
Although live monitoring is much better now and he sounds worse than ever, but that can be age.
He certainly was never a good singer though!
I saw an interview with the singer from The Seahorses and he said he also struggled live because John Squire was so loud on stage that he could barely hear himself.