Hello - please see attached. In reviewing the documentation, a single-channel interpolation use case is not well documented, and we are discussing internally on how to better articulate this and use the tvalid/tready on the sink side. Currently, the documentation states that it is a pass-thru from the tready on the source side (which is how it is behaving).
If you change your input to align with attached image, it should behave better. This would mean modulating the tvalid input to assert 1-in-80 clock cycles. If you receive in bursts, you should FIFO to holdoff.