SFLNXTCHG head-scratcher

It's been several years... meh, a decade since I last worked with subfiles
prior to this job I started a few months ago.

Have a situation where SFLNXTCHG is not working as I expect it to.

DSPF:
     A          R MAPTRA1S                  SFL
     A  39                                  SFLNXTCHG

RPGLE:
     FMAPTRADF  CF   E             WORKSTN INFDS(INFDS)
     F                                     SFILE(MAPTRA1S:RRNSF1)
     F                                     INDDS(DS@INDDS)
...
     D DS@INDDS        DS
     d SflNxtChg              39     39n
...
      *** A field defined in the subfile record is updated by the program,
then...
     c                   Eval      SflNxtChg =
*on                              SFLNXTCHG (for READC
     C                   UPDATE    MAPTRA1S
     c                   Eval      SflNxtChg =
*off                             SFLNXTCHG reset

--->  At this point, should a READC pick up the record that was just
updated?  In the program I'm modifying, the next I/O operation on the
workstation file is a READC on the subfile.  Does there need to be an
intervening EXFMT / WRITE of the subfile control record?  I tried a "1
SETLL  MAPTRA1S" after the update above, but the compiler isn't buying it.
(Also tried *LOVAL in factor 1.) Here is the code with the EOF test:

     C                   READC(E)  UGLTRA1S
     C                   DOW       %EOF = *OFF

Given the earlier code, I would have expected the READC to find the updated
record, but debug is showing that %EOF is *on, and is skipping the DOW
group.

Ideas, advice appreciated!

- Dan