ISTJs have a very loyal and straightforward personality. ISTJs won't cheat on you. They'll tell you directly if they can't continue the relationship. Due to different perspectives on relationship, a lot of possibilities for the relationship with ISTJs did not do very well.
ISTJs may told that their already give up on that relationship just to make sure the relationship not become more toxic or painful for them. ISTJs only avoid you when they want to avoid conflicts or sadness.
You know? ISTJs hate the "unsure" things that may waste their time. You need to be clear with an ISTJ about the relationship. Always remind them since ISTJ always forget about how their feel.
ISTJs can changed their emotions between happiness and sadness very quick. As ISTJs, I easily forgot the reasons why I feel mad or sad after 5 minutes.
Cheating and lying are two different things for me. Cheating only happen when a third party already involves when both of you still in relationship.