That is a "Display Title" in Foobar isn't it? Which is actually a concatenation of various tags into a string for display? If so, the actual sort is defined separately, and so I guess the image doesn't show the real basis of the sort.
Also, the definition of "Track #", known as %tracknumber% in Foobar is;
%tracknumber%
Two-digit index of specified track within the album. Available only when "tracknumber" field is present in track’s metadata. An extra '0' is placed in front of single digit track numbers (5 becomes 05).
How did you fit "A11" into a two digit index?
The alternate tag is %track number%, which is;
%track number%
Similar to %tracknumber%, however single digit track numbers are not reformatted to have an extra 0.
Which also wouldn't hold the value "A11".
Sorting in Foobar also seems to be defined by concatenating tags together, such as;
%ALBUM ARTIST% - %DATE% - %ALBUM% - %DISCNUMBER% - %TRACKNUMBER% - %TITLE%
I don't use Foobar so don't know the ins and outs, but it seems that Foobar is using Disc # and Track # in exactly the same way MC is, which is the industry standard; Numeric values. Maybe it is doing something tricky that I didn't find in my quick Google. But I could create displays such as shown in Foobar using a custom field as described above, and existing functionality.
This thread is a bit old, but it is discussing exactly the issue you have, and the proposed solutions for Foobar are exactly as we have described above.
https://hydrogenaud.io/index.php/topic,49594.0.htmlSo, has Foobar added functionality to handle vinyl sides since then? Do you have a reference you can share?
Here's another from 2015. Same problem, same solution. Track numbers are numeric and so sort correctly.
https://www.reddit.com/r/foobar2000/comments/36w2p1/anyone_else_organise_by_vinyl_side/I haven't found anything that suggested Foobar would accept alphanumeric values in %tracknumber%.
Another example for Logitech Media Server in this case, from 2012. Same issue. Same solutions. Track Number is numeric, because it is an industry standard.
https://forums.slimdevices.com/showthread.php?98935-Is-it-possible-to-sort-by-combined-alpha-numeric-combination&s=a8d350531ce8261ee87edb164d3e57adI haven't looked at Tau in detail, because I've done enough Googling for you. I wouldn't be surprised if it was exactly the same situation as Foobar.
If all the examples I could find for the same issue are using the same solution, why can't you use that solution in MC?