This exact thing happened to my tank. My tank is now 1 year old this week and is just now starting to not require as much alkalinity supplement. My tank is a 65 gallon system and was at times requiring 120ml of soda ash per day! It would drop from 8dkh to 6.5 in a day. If I left it alone it would stabilize around 5.5 or so. I did have some corals so was trying my darnedest to keep the levels up. I thought that was crazy so asked the same question as you. Some explained it as part of the nitrogen cycle. Some others claimed that it was because I used dry rock and sand and the rock and sand were absorbing it.
When I would test Calcium and Magnesium those levels were fine so I wasn’t really dosing them much because some other reefers said that it was okay to only dose for Alkalinity and not the others to keep proper parameters. However, I decided to just dose calcium and magnesium anyway even though levels seemed fine. Then I finally noticed that the Alkalinity wasn’t absorbed nearly as quickly. Maybe try dosing some Calcium and Magnesium just to see what happens to your Alk consumption.
After about 5-6 months I ended up getting a Trident and that really changed everything for me. I know can keep my dosing super dialed in. I’m now dosing about 60ml of Alk and Cal per day. I do have quite a few corals now.
Again, I’m still new to the hobby so take my advice with a grain a salt but just thought I’d throw my experiences out there to ya. Good luck and have fun
