The floor heating water separator has a sliding line corresponding to a small valve in each room. The property manager opened it with pliers and said it was okay. Does it matter?

It doesn't matter. Generally speaking, the valve of the water supply pipe on the water separator regulates the flow, and the valve on the return pipe is a switch. It is estimated that according to what you said, the bad flow control valve will be fully opened at most, which will not affect the temperature rise, and may affect the waterway balance a little. To put it simply, this room is small and doesn't need much hot water. The valve is half open. Now it's fully open, it will heat up faster and grab water from other rooms, but it's not a big problem.