Rapid or heavy frost buildup on a Liebherr freezer almost always points to the defrost system — either the heater itself (DF) failing to melt scheduled frost, or an evaporator sensor (F4) whose problem is allowing ice to accumulate faster than normal. Both produce a similar pattern: temperature looks fine at first, then gradually creeps upward over days as the ice blocks airflow around the evaporator.
A one-time manual defrost can offer temporary relief, but if the underlying heater or sensor problem caused it, frost will build back up on roughly the same schedule — the actual failed part needs to be identified and replaced to resolve it for good.