
If relevant, tell us what platform you're using the SDK on (Mac, Windows, Linux)
Mac 
Explain what the expected result was and what happened instead.
Expected arc to be drawn with a smooth line, but it appears to be cropped at times then jumps ahead of where it should be. It's only really evident at narrow line widths. 
Provide details on when and where this bug happens. Videos and images of the issue can be helpful.
Consistently.

Instructions on how to reproduce the issue.
The demo code below will produce it in the simulator. I don't have a device to test it.
local gfx = playdate.graphics
import "CoreLibs/graphics"
gfx.setColor(gfx.kColorBlack)
local x, y = 200, 120
local radius = 40
local startAngle = 0
local endAngle = 90
local linewidth = 2
function playdate.update()
startAngle +=1
endAngle += 1
gfx.clear()
 this looks fine (?)
gfx.setLineWidth(20)
gfx.drawArc(x, y, 20, startAngle, endAngle)
 this is broken
gfx.setLineWidth(linewidth)
gfx.drawArc(x, y, radius, startAngle, endAngle)
 this is fine
gfx.drawCircleAtPoint(x, y, 3)
 this is fine
gfx.setLineWidth(1)
local innerX1 = 50 * math.cos((startAngle90) * math.pi / 180) + x
local innerY1 = 50 * math.sin((startAngle90) * math.pi / 180) + y
local outerX1 = 100 * math.cos((startAngle90) * math.pi / 180) + x
local outerY1 = 100 * math.sin((startAngle90) * math.pi / 180) + y
gfx.drawLine(innerX1, innerY1, outerX1, outerY1)
local innerX1 = 50 * math.cos((endAngle90) * math.pi / 180) + x
local innerY1 = 50 * math.sin((endAngle90) * math.pi / 180) + y
local outerX1 = 100 * math.cos((endAngle90) * math.pi / 180) + x
local outerY1 = 100 * math.sin((endAngle90) * math.pi / 180) + y
gfx.drawLine(innerX1, innerY1, outerX1, outerY1)
end