It depends. But, if the decision to lower your rent was based upon a reflection of market conditions/due to the decrease in the market (as opposed to COVID-related financial distress) then generally, no, they cannot. Even if the landlord’s decision was not based on market factors, they may be prohibited from raising your rent to market rate based on other factors including whether your unit is subject to local or statewide rent caps.
The article here has an interesting discussion about this topic and includes information about the statewide anti-price gouging law but fails to mention statewide rent caps as an additional limit on a landlord’s ability to raise rent to market rate.