On my Roku Ultra 2020 the Peacock channel (menu and programs) are zoomed in cutting off right and bottom sides maybe 25%. The problem is not occurring on any other channels. BOTH the Roku ultra and the peacock app say they are current
Press "*" and use "Direct" under "picture size". If that does not work use "Auto". If neither of those options work (and this is highly unlikely since you say other apps work (though not what those apps are, so they may be 4:3 only), then your TV is incompatible.
This is a Roku Ultra 2020. Pressing the * does not bring up any Video options. The Video settings in the settings menu are only for HD or UHD resolutions (720,1080, 2160). No screen ration options. My TV is an LG OLED65E6P which supports all these resolutions (and others), native screen ratio is 16x9 but can accept 4x3. I did check the TV's own zoom controls to make sure it wasn't doing the Zooming.
Apps that work correctly for me include Roku Channel, Prime Video, VUDU, DIsney+, Broadway HD, YouTube, ABC, CNN Go, CBS All Channel Access, Fandango, Xumo, and HBOmax.
On my TCL Roku it just did this on Peacock during the OBS coverage of the pairs skating long program right before the Americans took the ice. It switched after a commercial and zoomed into the upper left quarter of the screen.
I got out of zoom mode by returning to the Roku home page. I flipped through settings but didn't find anything useful looking. Reopened Peacock, reopened the skating and it was fine.
My suspicion is a commercial was in 4k and it confused the 720p decoder.