This is complicated by the fact that earlier grants tend to be from when stock was worth less, so the are a comparatively high number of shares. So your comp will gradually inflate over 4 years because the value of the stock has gone up, meaning you are taking home more than the company 'intended' to pay you.
https://www.lastweekinaws.com/blog/aws-compensation-explaine...
I guess I am trying to understand whether this is causation or correlation relationship.
When you get promotion your base salary will increase, your bonus will also increase (due to being % of base, but also higher levels have higher %) and your annual stock refresher will be higher, as you are now on new level.
She would get annual stock refresher even without promotion, but with promotion refresher would be bigger and total new comp could have compensated for drop after initial grant has finished.
You can check levels.fyi for comparisons of total median comps between levels at Google.
L3 - $189k[0], L4 - $265k [1], L5 - $350k [2]
[0] https://www.levels.fyi/company/Google/salaries/Software-Engi...
[1] https://www.levels.fyi/company/Google/salaries/Software-Engi...
[2] https://www.levels.fyi/company/Google/salaries/Software-Engi...
So if you were granted 400k in stock over 4 years, you'd get 100k per year in stock (which you can keep or sell, but it's real compensation).
After those 4 years, the company will either give you another stock grant over another 4 years, or they will decide to show you the door - and not offer you anything.
That's what seems to have occurred here.