I will say that this is something I have experienced myself, long ago, so I do think it's likely there has been a bug report about it in the past... but the only things I can find (in a quick search) are the following:
First I found this:
Editing within grace period no longer remembers previous edit reason?
This was asked by current staff member and former mod animuson, prior to being staff. In this situation it seems that the bug was actually that the edit reason was getting longer every time there was a new grace period edit, which was bad. I agree that was bad and that fixing it was a good thing. That post led me to the original Feature Request that likely created the existing behavior:
Bring up original edit summary when editing your prior edit
Essentially, someone editing a multiple times within five minutes presented the editor with a blank field, which replaced the prior edit summary presumably with the default edit text such as "added 520 characters in body; edited tags; edited title", so we changed the system to retain the edit reason from the prior version of the edit:
The previous edit comment will now be retained when:
- an error has been detected during an edit submission
- the current user is quick editing within the 5 minute threshold
- anyone is editing a pending suggested edit
This will be deployed tonight or tomorrow.
This feature change also makes sense... overriding what might have been a really clear edit reason with the generic character count can be detrimental to understanding the edit histories. That said, it seems that testing indicates that this doesn't prevent the editor from overriding the edit history and there's nothing in Jarrod's answer (quoted) that indicates that editing the reason isn't possible or won't be updated.
Unfortunately, even in cases where there aren't actual grace period edits, this seems to have some negative impacts - Auto-generated edit reason being propagated from earlier revisions
Assuming Martijn's answer there is correct, it seems like pulling the existing edit text if the edit button is clicked within the grace period can mean that the edit reason is actually kinda useless if the edit is submitted after the grace period window, but this variation of the bug would be difficult to address and not really related here.
A possible explanation I found was this post, which was from 2013: Take most recent (or concatenate) edit comments when editing during grace period
In this case, the user made a grace period edit and only updated the edit reason (and not the post itself), which caused the edit to be discarded by the system. I think that when I've experienced this, it's likely for this same reason. The answer there recommends allowing edits to the edit comment to be saved if submitted in the grace period. This seems like a reasonable idea.
Now, you might be saying "but I did edit the post, I changed an image"... well... yeah... but what I'm seeing from your first edit (revision 8) is that it didn't seem to actually change the image URL - which leaves me wondering whether the edit actually went through or not.
If you know for certain that the updated image was added to revision 8 rather than only in revision 9, which is what I'm seeing, I'm not sure what's going on. That said, it does appear that the edit history has been redacted (appropriately) by the mods to remove the PII, which includes your revision 8. Your revision wouldn't have (shouldn't have) been included in the redaction if it didn't contain the image with the PII, which leads me to guess that it did still contain the original version of the image when the redaction occurred - which would imply that your grace period edit to update to the redacted image didn't go through properly.
Considering we can't reproduce the issue, I'm going to say this is my best guess of what happened but it'll be really difficult to know for certain and there's a lot of oddness involved. If you can reproduce this in a situation where an edit to the post goes through and the changed edit reason does not, let us know.