Or better yet injected into the plant a few weeks before cutting it down..
Agree - Glypho is so widely used that folks often misuse it - and a significant part of it's methodology to kill the plant relies on the plant continuing to grow (IIRC) - I think it makes it's roots burst or similar. So applying after all foliage cut (as folks often do) can still work but is less than ideal.
That Treeson looks like if used properly it should have done the job:
https://specialistsales.com.au/shop...re-treeson-herbicide-triclopyr-picloram/Is similar to the Yates Tree + Blackberry Killer.
You could either just ignore them, low and the grass will outcompete the myrtle eventually. Or break by bending each pup when it comes up and apply a little herbicide to each one, giving a few days before you mow - eventually again it should kill the underground remnants.
With patience I'm sure it'll go soon enough, can just take a lil while with tricky ones like Crepe Myrtles.