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%,''))"
"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.