It's never been a stellar focusing lens
Great optics at a great price, this lens has always been known for it's poor AF performance. From DPReview:
The big problem with this lens, however, lies in the focusing systems. The autofocus motor is relatively slow and noisy, which puts it at a major disadvantage compared to its ultrasonic motor-equipped competitors, simply because AF performance impacts strongly on many of the typical uses for a fast telezoom lens. The slow speed of the motor means focus tracking simply isn't fast enough for sports or action, or even children running around playing; it's also a distinct problem in low light or with low-contrast subjects, where the lens can take what feels like an eternity to achieve focus. This is compounded by the noise the motor makes, which could be a real problem in situations such as wedding or event photography for which the user wishes to remain unobtrusive; discrete it is not. Also the Tamron lacks a focus limiter switch, so has a tendency to hunt back and forwards through its entire range when it fails to lock focus first time. But perhaps the biggest problem we encountered was a tendency for the lens to mis-focus, seemingly at random and disturbingly frequently, such that F2.8 shots in particular were often not critically sharp (although it must be pointed out that this may be specific to either our test sample, or the Canon mount version).