Created actions just disappear with no warning

I just made the mistake of trying to create 5 actions at once and when I clicked close after verifying that all 5 were in the list in the Actions dialog, they all disappeared with no warning or explanation when I clicked close. They don't show in the Actions menu and they're not there when I reopen the Actions dialog.

I tried recreating just one of those actions, again clicking close once I verified it appeared in the list, and it also disappeared. An example action is below. Is there something wrong with the actions I am trying to create?

Action:
Name: MV-elec\_AA\ALB(YR)\#-AR-TR
Format value:
filename
$validate(X:\_sorted_electronic\
%albumartist%\%album% (%year%)\%track% - %artist% - %title%,-)

If there's something wrong with the action I am creating, is there perhaps a better way you could be failing here than just silently deleting it?

I did further digging and it looks like there is some correlation to the titles of the actions and the mta files they are individually stored as. I also saw you used INI files for some of your other data (genres, columns) but for some reason the actions are broken out into special files with the titles correlated to the filenames of the mta, which means that all the limitations of names in the filesystem also apply to action titles.

I also saw another post where they were adamant that the mta files not be modified directly because of some special formatting required to make them work. I'm not really sure what the point of having separate fragile files is if you can't do anything to modify them without risking breaking the application and it causes action formatting to be limited in ways it wouldn't be otherwise. Seems like another single INI with the titles specified within the file would have been better.

I ended up using the following title for that action instead.

Move#elec∕_AA∕ALB(YR)∕NUM-TT

I figured out from this process that I can't use the # symbol in the name since it turns out that it means "create sub actions" with a nested menu in the Actions dropdown. I also used the unicode character for division (∕) instead of a forward slash (/) since it seems to be valid in filenames, and looks enough like a path marker for my purposes. It'd still be nice to see some text validation done in the UI with a warning shown to the user rather than just having the action disappear when the title is invalid.

I do not think that the actions have disappeared altogether but that you will find them in a subfolder to the actions folder in MP3tag's data folder.
I agree, that it would be nicer if characters like the backslash that lead to subfolders and therefore break the conventions for action names would be eliminated so that a valid filename would be the result.
So check the %appdata%\actions\ folder and see if there are any subfolders. THen move the files that you find in the subfolders to the actions folder and you will see the actions in MP3tag again after a restart - which means that your work is not lost.

Did you SCROLL to the very bottom of your ACTIONS list. If you create one, but dont use it, the program dumps it to the bottom of the list.

I do not think that the actions have disappeared altogether

I had already checked for this. No additional MTA files are created in the folder %appdata%\Mp3tag\data\actions when an action title is specified that uses a character that is not valid in the filesystem. No subfolders exist in that folder.
As a test, I tried to create an action with the title abc\123 with a case conversion action on the comment field. When I click close, no additional mta file is created in that folder and no subfolders are created. This also means that when I go back to the Actions menu or the Actions dialog, the action I just created does not exist.
As I stated previously, when I attempt to create an Action with an illegal character, it silently fails and the action disappears without warning.

Did you SCROLL to the very bottom of your ACTIONS list.

Yes, of course I looked at the whole list. This should be simple enough to reproduce on your end following the steps I described above if you think I am making this up.

You are right.
I tried to create an action group with the name new\test.
MP3tag allowed me to enter whatever action I wanted.
The name new\test even appeared in the dialogue "Actions" (as the last action).
But when I tried to execute it via the menu, the action was gone.
A check in the folder data\actions and also in the dialogue "Actions" confirmed that the action group was gone.
I think this is a bug.