Why the lender's view is separate from the survey
A surveyor reports on the property. A lender decides whether it will lend against it. Those are different questions, and a report that reads reassuringly can still leave a lender's conditions unmet.
Lenders' requirements on knotweed are set out in their instructions to your conveyancer. They usually turn on documents rather than opinions: what treatment was carried out, by whom, what guarantee exists, and whether the valuer has seen it.
What lenders commonly ask for
The recurring elements are a treatment plan carried out by an accredited contractor, evidence that the work was completed, and a guarantee of a stated minimum term that is transferable and in many cases insurance-backed. The documents are then usually referred to the valuer for confirmation that the property remains suitable security.
Beyond that common shape, the detail varies: the minimum guarantee period, whether the guarantee must extend to mortgagees in possession, the distance from the boundary at which knotweed on adjoining land engages the requirement at all, and whether the issue needs to be reported to the lender in the first place.
Why there is no generic answer
Some lenders work from the categories in the current RICS guidance and only want to be told about the more serious ones. Some expressly identify circumstances in which the conveyancer does not need to report the issue at all. At least one will not proceed where the property is affected, whatever the arrangements.
This is why an answer taken from another transaction, or from general reading, is unsafe. The instructions that govern your purchase are the ones your lender has issued, in the form they currently take.