Test case is very simple:

 cat x.sh 
#!/bin/sh

[[ $A -lt $B ]]
[[ $A -lt $B ]]

$ ksh -R foo x.sh
x.sh: warning: line 3: -lt within [[...]] obsolete, use ((...))
x.sh: warning: line 4: -lt within [[...]] obsolete, use ((...))
$ grep obsolete foo
-5iIEV;v;Bsh: warning: line 4: -lt within [[...]] obsolete, use ((...))
1HFhH7;v;Bsh: warning: line 3: -lt within [[...]] obsolete, use ((...))

patch is also very simple:
diff -up ksh-20120801/src/cmd/ksh93/sh/parse.c.orig ksh-20120801/src/cmd/ksh93/sh/parse.c
--- ksh-20120801/src/cmd/ksh93/sh/parse.c.orig	2017-03-31 14:39:37.920062158 -0300
+++ ksh-20120801/src/cmd/ksh93/sh/parse.c	2017-03-31 14:40:14.200175096 -0300
@@ -2032,6 +2032,7 @@ unsigned long kiaentity(Lex_t *lexp,cons
 		else
 			sfputr(stkp,name,0);
 	}
+	sfputc(stkp,'\0');
 	np = nv_search(stakptr(offset),lexp->entity_tree,NV_ADD);
 	stkseek(stkp,offset);
 	np->nvalue.i = pkind;

The problem happens due to mixed usage of length based strings and
"C" (zero ended) strings. And from observation it only happens
after errors are reported. The patch just makes it work with
either string kind.