The default rates of decomposition in landfill for timber defined by the IPCC is actually set at 50%. So not all timber will completely decompose in a landfill. This is due to factors that inhibit decomposition such as access to air which landfills are actually designed to prohibit in order to keep it’s contents as inert as possible.
However, on-site studies have actually shown that the IPCC default decomposition factor is more likely to be an overestimation. You can access the study here. In addition, other studies even suggest that only 1% of timber used in construction will end up in landfill.
Therefore, even with worse case assumptions as per IPCC default estimates, timber will still show a net-negative result which further endorses timber as an incredibly low carbon construction material.