I don't understand this. I use "16:9" screen settings on the TV, as opposed to "Screen Fit" AKA "1:1" or "Pixel" (etc), which is the modern equivalent of overscan. Overscan was a technique in analogue broadcasting and CRT TVs to clean up the left and right edges of the picture (which would be ragged) and avoid displaying the non-picture frame lines at the top and bottom (they got used for Teletext data).
It used to be the case, although I haven't checked lately, that even with digital broadcasting the left and right edges were badly defined. On something like BBC Breakfast, which switches between multiple video sources, displaying it 1:1 resulted in the picture width jumping (but within the old overscan margin). Having the screen set to 16:9 (ie only displaying the non-overscan central area, with a few percent missing beyond the edges of the physical screen) avoids that width variation.
I have not noticed any captions going missing. Is that on start-up services that might not understand the broadcasting etiquette by any chance? They really should keep the critical content within the non-overscan area.
The screen 1:1 setting is for display use rather than broadcast TV, by which I mean as a computer monitor or perhaps DVD/Blu-ray.