Weird behavior with %REPLACE

Given the following block of code:

C CLEAR LFPOS 5 0 C EVAL LFPOS = %SCAN(X'0A':EVENTDTLX) C DOW LFPOS > 0 C EVAL EVENTDTL = %REPLACE('\n':EVENTDTLX:LFPOS:1)
C EVAL LFPOS = %SCAN(X'0A':EVENTDTLX) C ENDDO

and the following data in EVENTDTLX:

00000 C9409389 928540A2 9781944B 0A0AC940 - I like spam...I 00010 93899285 40A29781 944B0A0A C9409389 - like spam...I li
00020 928540A2 9781944B 40404040 40404040 - ke spam.

I get completely different behavior, depending on whether it executes in an interactive job or in a server job:

In an interactive job, it behaves correctly, replacing each hex '04' byte (an ASCII linefeed; not quite sure what it is in EBCDIC) with a backslash and a lowercase 'n', producing a final value for EVENTDTL of "I like spam.\n\nI like spam.\n\nI like spam."

But in a server job, the %REPLACE is evidently doing absolutely nothing, as if it's somehow treating the '\n' the way C would treat it (an escape sequence representing a linefeed), with the result that this thing tightloops.

Has anybody ever heard of such a thing happening?

The server program uses HTTPAPI to call the Google Calendar web services, while the test program I executed on a terminal makes no explicit external calls whatsoever.

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JHHL