Finally was able to get to it.
It does look like MC (now 30) does not pass the tags to lame. In the command line it has none of the tagging CLI options. Here is the configuration attached.
Adding hard coded values in the command line makes no difference, the fields are still "Unknown" upon the export conversion:
-V 4 --vbr-new --tt test --ta test --tl test
I am really curious on what your configuration/command line shown as?
So my settings aren't meaningfully different than yours, and it's working fine here on MC 30. I don't use the custom command line switches option, I just us the VBR "High" preset, so my custom command line setting isn't really relevant, but FWIW it looks like yours. Also, just to rule something out and I know this may sound dumb, but in your screencap, you don't have the custom command line switches option radio button checked, so none of those command line flags *should* be doing anything at all. You're using the built in quality preset, not the custom command line in that screen capture so the CLI options won't do anything if you change them. I assume when you tested the changes to the command line that you moved the radio button down to select the custom tags, right?
You might've missed one of my questions in my earlier reply: are the files you're converting imported in JRiver, and does the JRiver library have the correct tags as well? Like if you look at the FLAC files in JRiver, do you see the correct tag information? If not that's probably the issue (i.e. JRiver is writing it's database tags rather than the file tags). If all the tag data is correctly in your JRiver library there's something odd happening here.