I am sitting here freezing and you've been out playing in your garden? Life ain't fair! Grins. What zone and hemisphere of this planet do you live?
There is a bit of life evident for your lavender. I would pull this plant up, rip off the dead roots and plant material, plant the live remainder in a one gallon pot of sterilized potting soil.
Glyphosate works primarily by being sprayed or applied to vigorously photosynthesizing leafy plant material. It is then transported to the roots, killing the roots and then the entire plant in 3 weeks.
If you hadn't said glyphosate, your plant is displaying very normal lavender die out. Is this the only lavender? What other plants are in this bed? You will need to replace this lavender but it would be nice to know the new lavender will not have the same problem. Next year, if the bit of lavender you transplanted into a 1 gallon pot in just potting soil thrives, you will be able to add to a lavender 'mass' ??
So if there had been the tiniest bit of wind that day you sprayed, I'll bet your lavender got sprayed. This is called 'Drift'. It takes minuscule glyphosate sprayed on leaves to kill a plant.
Glyphosate is only advantageous when needing to kill a large area of weeds. Should never be used in ornamental plant beds or vegetable gardens. I only used this for gravel walks and driveways, gravel patios. Only in the spring after weeds have had a chance to get vigorous but before they go to seed. And on a dead quiet, very warm, still air day. Sprayer held down very close to the gravel. It only takes ONE drop of glyphosate (a big drop) to kill a plant whether weed or ornamental or edible. You do not have to spray till runoff.
Your assessment about your lavender is probably right on. Glyphosate drift. If you were thinking there was enough glyphosate sprayed that the roots would take it up I am thinking there was more than enough drift to kill this lavender.
Has it been 3 weeks since you sprayed? How much was sprayed? Did you soak the soil?