I have a friend who worked at Gusto, and my wife tried using Gusto for her small business (they handle payroll for small business, got a big boost from the pandemic). The lack of technical resolution here is so Gusto, it hurts.
That's unfair: isn't this is exactly how most strange bugs get "fixed" by most companies?
It is an abnormal developer and an even more abnormal business that actually spends enough time to find the root cause of outré glitches. Especially when you start having to debug complex third party systems to debug them properly - requires skills and motivation plus a company that will encourage a developer to do that.
The story is not specific to Gusto - it is the story of every developers life. I have chased down bugs in my OS and my browser - it is rarely well rewarded! Fixing a compiler bug should be on my bucket list! A long time ago I worked around a compiler bug by inserting a label: (I think the label prevented certain optimisations where the label was put).
I mean, this story is a hell of a rundown of debugging. The fact they don't have insight into the ways Chrome or Grammarly work isn't something to apologize for.