I recommend you stir your sand bed to check if it is calcifying. If it is turning to rock, that is where the soda ash is going.
What was your daily alkalinity consumption prior to using soda ash? With such a young tank, I would expect alkalinity consumption to be < 1dkH per day.
Do you trend pH with a controller/monitor? What is your daily peak pH after starting to dose soda ash?
What is your calcium level? Is it also dropping?
What is your magnesium level? Goal is to maintain it in a 3:1 ratio with calcium. Higher magnesium allows for greater alkalinity/calcium levels.
I have personally overfed Soda Ash before in a new tank. PH was sky high, alkalinity and calcium kept dropping no matter what I did. I was dosing 150ml/day in 55 gal of water. My sand bed quickly turned to rock.
If you don’t have another alkalinity source on hand, I recommend dropping your feed to approximately 30ml/day and monitor alkalinity.