Right, I mistested when triyng to replicate the issue outside of my big chunk of code.
The issue only appears when using the drawtext -p switch to support control codes.
//window -pdCBfo +l @test -1 -1 400 400 | drawrect -fr @test 16777215 0 0 0 400 400 | drawtext -rpn @test 0 "segoe ui symbol" 50 100 100 $+(A,$chr(9),B) | drawdot @test