Sometimes in the web application you will be reassigning a supplier, it will revert back to the original supplier allocation.

This is likely due to the supplier being locked to the task by a Purchase Order or template rule.

In the management node for the job, under Unlock Supplier, can you select the Task and at the bottom of the list press Unlock Supplier.

If you have Purchase Order integration you should also make the update in your purchase order system to avoid incorrect data, or possibly creating a PO Split task in ClickHome when the PO re-integrates with the old supplier information.