I’m thinking if you had been using this as a streaming device, you probably have it connected to power constantly? This is a conversation I frequently have with customers at the shop…battery health degrades (with time and with use) to the point where it starts to become noticeable, and as a consequence, people tend to leave the computer connected to power, which accelerates the decline and causes this problem.
OWC has reasonably priced replacement batteries for these MacBook Pro models, and very detailed guides on installation…but the process is somewhat complicated, and not for the faint of heart.