I still wonder this myself, having struggled with a different issue (since fixed) with HLA formatting:
I’m still guessing that it’s Handlebars JS template engine (or a compatible/similar syntax), which internally formats the HLA python data for GUI display. I understand the Logic 2 program itself is based on the Electron framework – which uses the chromium display engine for the GUI (typically implemented using Node.js and other JavaScript).
Meanwhile, if you want more control of the display text, you might try converting python type from bytes to a more compatible data type in the HLA python code before returning the value vs. directly returning a bytes typed object (which isn’t as cleanly passed between python and JavaScript)?
Finally, perhaps Saleae could confirm what specific syntax the HLA format string is using?