Window split didn't copy the value of 'conceallevel'.
diff --git a/src/option.c b/src/option.c
index 8d7f09a..8a3f6b5 100644
--- a/src/option.c
+++ b/src/option.c
@@ -9766,6 +9766,7 @@
 #endif
 #ifdef FEAT_CONCEAL
     to->wo_cocu = vim_strsave(from->wo_cocu);
+    to->wo_cole = from->wo_cole;
 #endif
 #ifdef FEAT_FOLDING
     to->wo_fdc = from->wo_fdc;