This was fixed in 5.17.3 by the following commit:

commit 34b39fc9cd81fbff0d52451a5c4570293817ca32
Author: Karl Williamson <public@khwilliamson.com>
Date:   Thu Aug 9 14:38:03 2012 -0600

    regcomp.c: Set flags when optimizing a [char class]
    
    A bracketed character class containing a single Latin1-range chara
+cter
    has long been optimized into an EXACT node.  Also, flags are set t
+o
    include SIMPLE.  However, EXACT nodes containing code points that 
+are
    different when encoded under UTF-8 versus not UTF-8 should not be 
+marked
    simple.
    
    To fix this, the address of the flags parameter is now passed to
    regclass(), the function that parses bracketed character classes, 
+which
    now sets it appropriately.  The unconditional setting of SIMPLE th
+at was
    always done in the code after calling regclass() has been removed.
    
    In addition, the setting of the flags for EXACT nodes has been pus
+hed
    into the common function that populates them.
    
    regclass() will also now increment the naughtiness count if optimi
+zed to
    a node that normally does that.  I do not understand this heuristi
+c
    behavior very well, and could not come up with a test case for it;
    experimentation revealed that there are no test cases in our test 
+suite
    for which naughtiness makes any difference at all.
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Dave.