Having a splitter does not reduce the signal by 50% and is not an issue if RF is properly checked. You can have too much signal and a splitter is sometimes used to attenuate the signal to a cable modem. It’s all about proper RF levels. Splitters are okay.

As someone who has been in the coax/cable industry for 17+ years, this can be a number of things. If you disconnected and nothing happened, it may not be active with RF. Usually when we see a splice like this behind a wall plate, it means it’s looping to another outlet somewhere in the house/unit. Only way of knowing is toning the line out or using a meter to check RF levels. Not an ideal way to wire this day and age.

I know this is not a straight forward answer, but the easiest thing would be to call your ISP and have them move the modem and/or test RF signal. You may move it and it works, but the RF may have poor signal and cause service impacting issues.