What happened to the Galatian congregations when Paul was not physically present?

When Paul was absent, false teachers entered like ravening wolves and tore the Galatian congregations apart. The Judaizers persuaded them toward circumcision and law keeping, pulling them from the pure Gospel of Grace into a perverted gospel that troubled them.

Les explains that while Paul was physically present, the Galatian believers could hold things together. But once he left, false teachers came in like ravening wolves and tore the little congregations apart. They began to believe Judaizers who demanded circumcision and law keeping, turning from Paul's pure Gospel of Grace to another gospel, which was not truly another but a perversion. Eventually the perverters evidently succeeded, so that all in Asia, including Galatia, turned against him.

Galatians 4:18

But it is good to be zealously affected always in a good thing, and not only when I am present with you.

What does Galatians 4:18 mean about being zealous only when Paul was present?

Galatians 4:18 means zeal for the good thing should be continual, not only when Paul was physically present. The Galatians held together while he was with them, but once he left, false teachers entered like ravening wolves and tore the congregations apart, showing their zeal depended on his presence.