This is not a "work around" but the totally general error free standard behaviour since decades of computer operation.
So your bug report sounds to me like misunderstanding and misuse by user.
Regardless what input operation is currently in use, clicking on the "Save" button always triggers Mp3tag to save the current temporary working copy of the entire tag back to the file and returns a status message.
As you have described above your edit work was not completed before pressing the "Save" button.
In other words ... your input was not taken over into the temporary working copy of this tag field you was working on. The blinking cursor demonstrates that no change to the tagfield has been made durable yet.
But once you do "enter" the data into the tag field Mp3tag takes knowledge about the change and is able to use it for the next saving process.
You should be aware that there is also a checkbox option in Mp3tag/Tools/Mp3tag Options/Tags to enable/disable auto saving when leaving a tag field.
But this option in fact cannot do anything against misuse by user.