Yup, these can be very frustrating.
I always
search through and pull out any coins like this
before doing any water soaks as even that can harm some.
I then gently poke and prod with a toothpick to see just how stable, or not, the remaining
patina is.
If fairly stable, then one can often clean off some dirt with soft toothbrush and toothpick, or even metal pin in some cases, then wax and store.
If unstable, then there is little that can be done. Sometimes rubbing the coin gently between thumb and fore-finger can
help - believe it or not. Waxing can
help, if it can stand it, as it reduces friction on it later. Such coins also often do better in a tray than in a
flip or envelope.
SC