If I try to replace some filed completely and to use regex ".*" to be replaced with something new. It will display the replacement twice. For example,
IN ARTIST field,
I use Actions -> Quick Actions -> Replace with Regular Expression.
Regex: .*
Replace: ReplacementText
It will be changed to ReplacementTextReplacementText instead of just ReplacementText
Also, if the field is is blank. This regex does not make any changes.
If you want to set a new text for a whole field, use an action of the type "Format value" for that field.
BTW: there are no "empty" fields. If there is no data in a field then the field is not there.
First of all, thanks for taking the time to report this. It's not a bug, but a peculiarity of the regular expression you've used.
.* matches any character between zero and unlimited times, as many times as possible. So it first matches all characters in your string and then an additional match for the zero characters at the end. I know it seems counter-intuitive, but this is how the * quantifier is specified 
As pointed out by @ohrenkino, Mp3tag has not concept of blank fields — those are simply not present in the metadata and you can use the action type "Format value" for, e.g., ARTIST to set the field to ReplacementText.