yes the Mazda UA and F2 series engines were as you call it "safe engines". The valve clearances were fairly wide so the chance of valve damage was small, not non-existent though, if the belt was to break if the engine was at high rpm's valve damage can occur. Most of the time it doesn't because the belt usually breaks during start-up.

hopefully you checked the idler tension pulley also and replaced that if need be, they can cause this to happen if they lock up from bad bearings.