Handling of the tag panel and the files list is different - and I assume that you did your editing in the files list.
Editing in the files list immediately saves the modification as soon as you leave the (editable) field. But not before.
Ctrl-S (or the save icon in the tool bar) write back the data from the tag panel.
So if you are in the state of an unfinished editing process (i.e. you have not left the field where you modified the data), Ctrl-S saves the contents of the tag panel in the file - this leads to the effect that unsaved modifications get overwritten.
If you do not use the keyboard shortcut but click on the tool bar icon, you will not see this effect as you then have left the field in the files list which saves the modification.
Why does editing in the tag panel behave differently?
The tag panel by design allows to edit information for all the selected files - this is not possible in the files list where editing is only possible, if a single file is selected.
So if leaving a field in the tag panel would trigger MP3tag to save the modification, then this could considerably slow down the editing process as the modification from one single field would be written to all the selected files.
Instead you have to tell MP3tag that you have finished editing (all of the fields) and the modifications in all fields get written to the files in one go. This is quicker.
My contribution was meant to be a description of the current function that might shed some light of the possible motivation why MP3tag behaves the way it does.
It does not judge whether this is a bug or not.
The description of Ctrl-S in the help does not say anything more than just "Save tag", yet MP3tag behaves like "Save data in tag-panel to tag" https://docs.mp3tag.de/keyboard
Sorry for the spamming @_@ it would be helpful not to be on mobile right now.
I fully understand where you are coming from on a technical standpoint but my reason for bringing this up mostly was because when using Ctrl-S in the Filename column it will automatically push the change and save it to the file. Where as any other column won't which makes the feature a little confusing when it works in one specific place but none of the others
The Save-button, Save-Tag and CTRL-s only produce a result to changed tags in the columns, that are not in the tag-panel.
If you define a tag-field in the column-view that ist not in the tag-panel CTRL-s works as you wish.
I doubt that this has anything to do with the filename but whether that field is actually displayed in the tag panel or not.
Please take any other field in the files list that has no representation in the tag panel and press Ctrl-S and you will see that the modified data will be saved - and not the previously present piece of information.
As soon as you leave a field in the files list (and the modification is saved) you will see that the field(s) in the tag panel get updated - and so only the uptodate data will be saved if you press Ctrl-S.