guillotine stem cutter

Make sure your tires are the right pressure.

Are you running Presta or Shrader valve tubes? If Presta, is the rim meant for Shrader? Are you running with rim strips properly placed? Have you sanded the valve hole rim with a fine grit paper? Are you using the same brand tubes every time?

Previous replies are correct. Over 15 years of bike shop experience and virtually every cut valve I have seen was due to improper inflation. A few things that can have you chasing your tail though so check these also to be safe. Make sure there is not a defect or tear in the rim strip or more common is a tiny thorn or piece of wire stuck in the tire either can cause a slow enough leak that you properly inflate then head out on a ride just to have the tire loose enough air that it cuts the valve from shifting on the rim. Also as mentioned a presta tube will cut in a schrader hole without a grommet or a bunch of tape. Just 're read for clarification if it's hot it's from braking and almost 100% the tire shifting on the rim.

I've seen several aftermarket tires that would creep on the rim when underinflated with motor power leading to a cut stem. Keep that pressure right!

I agree there.... and have also seen countless bikes with the valve stem crooked and not extended completely thru the rim. This leaves it vulnerable to be cut thru by the wheel opening since now there's also the piston pulses exerting some force on the wheel valve stem opening to the valve stem, making it vulnerable to be sawed thru. This isn't quite as bad of an issue from normal pedal power but still not good.
It only takes a few minutes to make sure the valve stem is centered and fully extended thru the wheel opening. These do tend to get pushed in and stay in during inflation, especially when a tire chuck is being used since the valve stem does get pushed on pretty hard to get the air into the tube but one could hold the valve stem with pliers or use a bicycle pump with a lever lock on the stem to prevent it from being pushed in during initial inflation. The presta type valves have a small nut to keep them from being pushed in but the shader valves need to be held out so they don't get stuck half way in the wheel opening.

What I have done to keep the valve stem centered is simply hold the valve stem and pump end (not sure if there's a technical term for that or not) to keep it straight until I have it pumped to about 20 psi (pump has a gauge on it). What I used to do was pump the tire up and if the stem wasn't straight I'd deflate, try to center it and pump it up again. That worked Ok, but was time consuming. Holding the valve stem while pumping works better and takes about the normal amount of time to pump up a tire.

You can get tubes with metal threaded valve stems.
I use Specialized ones they work great.
Don't tighten the stem nut until you air up and seat the tire bead.