Same problem here. Roku stops streaming like clockwork every 3 minutes no matter which app we are viewing (Pluto, Paramount Plus, etc.) It exits out to the Home Screen. We are also on Software version 12.5.0.
Update: We tested our Roku Premiere & Roku Express on our 2 bedroom TV’s (smart Vizio and non-smart JVC) and they work fine—no 3-minute drop off. Those same 2 Rokus consistently stop streaming after 3 minutes ONLY on our living room TV, which is a Sharp brand (non-smart).