This is one of the #1 things that affect small pianos. Not just japanese
You need a regulation. If a technician comes in from the store, and spends 5 minutes. Tell him to get back to work.
He probably will try to just adjust the let-off. (5 minutes)
This may treat the symptom, but it will not solve the problem.
The problem may be in other areas, including key depth, hammer stroke, jack alignment, backcheck alignment, or any other (of a million) thing. A piano should be comletely regulated, because changing a setting to overcome a symptom, will throw the geometry out, and cause future problems.
Any other problems, please PM me, if you need to check a quote to see if it is fair. (I am a technician) and I would be happy to interpret anytihing your tech says.