I would call Scalemaster in Stone and seek their advice. The level of water in the salt bin is normally determined by the timer controlling the Softener valve rotation at the end of the recharge process and not a level sensor. So there could be an issue with the Softener valve. If is does require a new valve, it is often cheaper to replace the complete softener. The softener valve is expensive and difficult to fit (achieve a water tight seal with the “O” ring to the catalytic vessel).
Alternatively the seal with the catalytic vessel may be leaking into the salt bin, causing it to increase in level after a recharge cycle. The way to test this is to recharge the softener, remove water with a mug until it is well below the over flow and mark the water level. It should not change - unless of course the softener recharges. If the water level increases it is probably a failed “O” ring seal between the valve and the vessel. To change it, requires the valve head to be removed. If that is the issue and the softener is more than 8 years old, I would change the mol sieve in the vessel at the same time. The “O” ring can be quite hard to reseal properly and will need lubricating with potable “O” ring grease.
Don’t leave it. If the overflow cannot handle any discharge and brine overflows into the kitchen - it is very difficult to get rid of the salt and can / will cause long term damage to solid floors, metallic objects, chipboard kitchen furniture et al.
