Quantity values of type Dimensionless can be a bit quirky.
Dimensionless does have units - it’s about ratios, so units % percentage and dB decibels can be used.
There is also the default unit, “1”, for a simple ration like 4:1, four-to-one.
So if you send say a Number:Dimensionless type Item a Quantity that gets interpreted as just “50”, that will be accepted as 50:1.
If you then display the same value but request in % units, you will see 50000%
So maybe this is going wrong. How are you doing this ‘writing’? What does your events.log show gets loaded into the Item state? How are you adding “%”, should you try +" %" with a space?
It might be a charting problem - do you get the other numeric displays you might expect?
Charts are quite likely to mess up in some editing situations, because persistence does not like it at all if you change an Item type after you have already started persisting it. That apply here?