From Tamron Japan (translated):
[Update] Firmware update for 18-400mm F/3.5-6.3 Di II VC HLD (Model B028) for Canon
Thank you for your continued patronage of Tamron products.
On our website dated October 25, 2022, when using our lens 18-400mm F/3.5-6.3 Di II VC HLD (Model B028) with Canon EOS R7 / EOS R10 via an EF-EOS R adapter, we posted a notice that the problem occurred, but we would like to inform you that it has been improved by updating the firmware.
Points to be improved
A phenomenon in which AF may not operate properly when the shutter button is half-pressed, depending on the timing of the camera's power ON/OFF.
How to update
It is possible to update with the TAP-in Console (sold separately). If you do not have a TAP-in Console, we will update the firmware (free of charge).
Click here for details >>>
Learn more: Tamron 18-400mm f/3.5-6.3 Di II VC HLD Lens Review
Get the Tamron 18-400mm f/3.5-6.3 Di II VC HLD Lens at B&H.