Explain the problem.
Adding characters to #+pandoc-emphasis-pre as described in the manual doesn't work as expected. Interestingly, adding to #+pandoc-emphasis-post does.

Minimal working example test.org:

#+pandoc-emphasis-pre: "-\t ('\"{T"
#+pandoc-emphasis-post: "-\t\n .,:!?;'\")}[t"

1. T/est/ with T allowed as pre
2. /Tes/t with t allowed as post
3. Normal /emphasis/, and in {/brackets/}

Command: pandoc -o test.md test.org

Expected test.md result:

1.  T*est* with T allowed as pre
2.  *Tes*t with t allowed as post
3.  Normal *emphasis*, and in {*brackets*}

Actual result:

1.  T/est/ with T allowed as pre
2.  *Tes*t with t allowed as post
3.  Normal *emphasis*, and in {*brackets*}

Exporting to Pandoc AST confirms that the problem is with the reader.

Also completely replacing the strings with "T" and "t" respectively achieves the same result.

Pandoc version?
Pandoc 2.18 on Manjaro Linux (pandoc-2.18-linux-amd64.tar.gz from the release page). Also happens on https://pandoc.org/try