Odd behavior on member save

Darndest thing.

I open a source member from a source file that has record length of 112. I
verified this both on the iSeries and via RSE right click properties on the
source file.

I key a comment line of dashes right over to column 100, which ought to be
fine.

I get "Line truncation occurred during the save" and the line lights up
(pink in my case). The member was saved but did hack off the bit of line
exceeding column 80 (which sounds suspiciously like the old 92 byte record
length files of which there are none on my system).

Huh?

I edited the exact same member in SEU. Keyed the same comment all the way
to 100. Saved just fine. Opened again in RDi, I get the truncation error
again.

I open a source member from a different source file different library,
record length 112. I key the same comment, all the way to column 100,
saves without issue.

It's not the language (it does the same with C, RPGLE, CMD, PNLGRP, and
XML).

Anyone have an idea what to check (perhaps cache-wise or temporary source
member-wise) that makes the two identical source files be handled
differently? I can see (in my server job on the i) a temporary source
member being created and copied and removed.

Stu