I came across this too, but never reported it as a bug because it is related to the style and your preference. I just replaced the xen:calc with the margin I wanted, TMS handles it with no issues on updates. I chose editing the template directly instead of setting the
@messageUserText.padding-left and the right to 0 because I don't use it in my current style.
I tried to reproduce it and couldn't either. At the time I didn't pay much attention to it because I figured it was just me and the way I chose to set some of the style properties. I just removed the xen:calc from the CSS, and replaced it with a fixed value.
Sorry, I couldn't be more helpful, but I do recall that the xen:calc did cause an error... maybe because I put another style property in the padding field instead of an actual value?