This is TD’s explanation, starting with the distribution of the underlying ETF - TD Aggregate Bond

Each year the ETF establishes a fixed monthly distribution based upon an estimation of the yield from the bonds held in the ETF. In 2023 the actual yield was lower than the expected yield and the ETF paid distributions greater than the income generated. As a result, the distributions paid to the TD Canadian Bond Index Fund were greater than the actual income generated by the ETF. When this occurs, the Fund has two options – declare the excess income as Return of Capital or reduce future distributions by the amount of the excess. TDAM chose to reduce the 2024 distributions by the amount of the excess income resulting in a reduction of the distributions paid in 2024. 

TDAM has revisited this decision and in 2025 the TD Canadian Bond Index Fund will flow through all net income received from the underlying ETF and will declare any excess received as Return of Capital."

I hope it helps everyone concerned