The only time I ever bother to close apps on my iOS devices is when I do Pimsleur language courses, played in the stock music app. I found that pause/play would introduce stutters and skips if I had a lot of apps open. Each course file is ~30 minutes long, atypical of songs. Reproducible over many tries and several iOS versions.

OK, I lied. The only other time I close an app… The. Chess.com app's analysis board becomes unbearably sluggish if the app has been running too long.

Both instances can be pinned on the app rather than iOS.