I recently suggested an edit for a tag wiki. When it was approved, the edit reason in the yellow box disappeared on the suggested edit page, although it still shows in the permalink revision page and the edit history for the tag wiki.
Just to clarify, the reason did show in that yellow box before the edit got approved.
Also, just a side-note: why are we not allowed to add some message when creating a tag wiki? The "adding # characters in body" message is kind of ambiguous and a tad misleading. Could it not at least say "created tag wiki with # characters" or something more descriptive? My friend was actually confused by that message since he's not as experienced in SO's workings. He thought the page was just a joke and I had to explain to him that it's just how it gets handled.