IsolationForest Decision Function vs. Anomaly Prediction Question
It looks that your PCA is not mapping well the data. I recommend you to look at other dimensionality reductions techniques, such as, UMAP, TSNE, in order to see if you can achieve better representation. If you do not have too much data, you could use also MDS since it will preserve local distances (however, computationally very expensive):

https://scikit-learn.org/stable/modules/generated/sklearn.manifold.MDS.html

If only the visualization is what your are after, then you could just simply run Isolation Forest on 2 Principal components, then the boundaries should be more obvious in the first picture. Of course, the actual results may be misleading.