When using the foliage tool, once the mesh count reaches around 1.5k and higher you will be unable to select individual meshes by using the mouse. The lasso tool and the select all tool work as intended, the mouse is the only one being affected. If the mesh count drops below 1.5k you will be able to individually select meshes again. This issue is a regression. This issue is similar to
4.16.3 - CL: 3561208 - Issue does not occur
4.17.1 - CL: 3586342 - Issue occurs
4.18 - CL: 3597901 - Issue occurs
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-48942 in the post.