Nokia India — automatic decrease of ringtone volume

I have purchased nokia 6 via flip cart in mar'18 and it ran good for some days

After releasing update 8.1.0 we had updated same and now the problem is that the ringtone which plays for an incoming call is ringing at the least volume after setting higher. Also when try to increasing the volume it is not increasing till the restart. Kindly suggest the solution of it we purchased nokia by the brand name and trust hope the trust will be continue.