Hairline cracking in section of painted interior wall
That's exactly what happens when painting over remnant wallpaper paste. Or, if a white glue was used on a wall repair.

The only remedy is to scrape the offensive stuff off, wipe several times with a wet sponge...until no longer slippery and re-prime/paint.

A skin-thin application of Spackle/plasterboard compound can restore the scraped areas to paint elevation beautifully before re-priming/painting.