I'm not sure I'm understanding you.

When you say you already have monikers set on some Comcode pages, did you set them through the UI?
When you edit them, are you saying the "previously-existing moniker" changes (aka. what was set when you added the page is changed to the codename, and then a new moniker is also created via the edit which is different from the first moniker [since it was changed to the codename])?

I don't know of another way to set monikers so yes. The pages were custom, then edited via the UI. Moniker set at the same time as updating other info and when saved the moniker is set to the codename (which is different in my case). Not a big issue, just a bit of an annoyance really. I think unless you create the page within Composr, it happens.

I cannot reproduce this issue on my Windows nor Linux machine. I created a page, codename "test" moniker "test-page", saved, edited content but left moniker alone, saved, edited again, and moniker was still "test-page" as expected.

I think we'd need to see a video showing the issue.