Putting the did -f on a timer 1 0 makes it work.

Code:
dialog TestTabActivate {
  title TestTabActivate
  size -1 -1 100 100
  option dbu
  tab "Tab 1", 1, 2 2 96 96
  tab "Tab 2", 2
  tab "Tab 3", 3
}

on *:dialog:TestTabActivate:init:0: {
  .timer 1 0 did -f $dname 2
}

alias TestTab dialog -mdh TestTabActivate TestTabActivate


So yes - an issue when run inside the Init event.