I saw this behavior in last (3? 6?) month. Win7x64hun, Mp3tag allways the last (beta too), foobar allways the last (beta too). Earlier (through years) scripts worked without problem.
When I send multiple files from foobar2000 (using Run services) to Mp3tag, not all files "landing" in Mp3tag's list, only some (sometime only 1) of them. Repeating 3-6 (!) times the operation in foobar usually resolves the problem.
Sending files from Total Commander I never saw such behavior. I think, it is associated with foo_run (and it is Florian's brilliant work, too - thanks for it!) or with foobar itself.
2 scripts in Run services are:
"C:\Program Files '('x86')'\Mp3tag\Mp3tag.exe" "$substr(%_path%,0,$strrchr(%_path%,''))"
and
"C:\Program Files '('x86')'\Mp3tag\Mp3tag.exe" "$left($directory_path(%_path%),$sub($len($directory_path(%_path%)),$len($directory(%_path%))))"
Using the second script (sending folders) the bug seems much less often, not all times. Using the first script (sending files) seems in 95% of cases.
But I see, in my first comment I did an error. Those 2 scripts are not for sending files, or sending folders, but for folders and parent folders. The right ones:
One file is not a problem. But when selecting multiple files for sending, not all of them are loaded to Mp3tag; and that bug doesn't seem to follow a pattern
[I've recently tried different settings if FreeCommander, even more than why I dealt with the Spek and Spectro issue, but It didn't helped]
Since then, I have not noticed the error. 99% seems to be 100%, so, Florian, in my opinion the original bug (foobar>>Mp3tag) now is fixed ([F]). And thank you for the correction.
@stevehero
Thank you for your tip. Sorry, it does not solve the problem, at least for me. "C:\Program Files '('x86')'\Mp3tag\Mp3tag.exe" "%_path%" sends only the first selected file to Mp3tag, whatever settings (wait, thread...) I use.
And, in general: any program without english help is not serious in my eyes. Japanese help file with google translator is not my best best friend... I gave it up.
But I have a new idea. Try to find previous version of foo_run, and test it.
...
Sorry, could not find it. @Florian, may I ask you to point to?
Stevehero, I think, previously you did not understand me exactly. Not (only) the help file itself is the point. If a public program does not have english help, it means, that its creator does not care about users. He is negligent. And such quality is the program itself. For such a program to spent a big time: error.
But, unfortunately: it repeats exactly the same symptoms, as foo_run. I selected 58 files in foobar, foo_run_group sends (tried 4×): 57, 7, 51, 52 files. But it helped (someone...) in diagnostics: from foo_run remained only common with foo_run_group elements (+foobar and Mp3tag, of course).
Later I shall diagnose it with clean fresh foobar installation, adding plugins one-by-one. May be, one of them conflicts with foo_run (and same others). Sending multiple files from Total Commander (or Explorer) through the shell works without problem.
Does not work, because of wait checkbox.
Without: sometimes work well, sometimes not. Same, as foo_run. Tried many times. Foobar refreshed from last stable to last beta, no difference.
Or, drag and drop.
-- but in most cases (nearly 99%). Big difference! 
