Why does my portable AC shut off the compressor after 30 seconds every afternoon?

The pattern you're describing points to the compressor cutting out on overheat. It runs 20-30 seconds, the thermal cutoff on the compressor trips, then it cools down and tries again a few minutes later. Morning works fine because the air going through the condenser coil is cool. Afternoon, the unit is sucking in hot room air to reject heat with, and the whole thing is running hotter than it can handle. Two things are stacking against you here. First, the exhaust setup. Those units are designed for a hose roughly the stock length with minimal bends. Every extra foot and every kink adds back pressure, less airflow through the condenser, and more heat staying inside the machine. If you can cut the hose down or reroute it so there's one gentle sweep instead of a couple of sharp turns, do it. I've seen a straightened hose be the difference between constant cycling and running all afternoon on these portables.

Second, the space itself. An uninsulated metal roof over 300 square feet in afternoon sun is a big heat load, and 10k BTU was never going to hold it at 75. A portable also has to use room air to cool its own guts, so the hotter the room gets, the worse it runs. It's partly undersized, but the undersizing shows up as it just never catching up, not as the stop-start behavior. The cycling is the unit protecting itself. Things worth trying: get that exhaust as short and straight as possable, prop open the window a hair beyond the seal if the hose runs hot to the touch, and if you can shade the roof or throw a cheap radiant barrier or even a tarp a few inches above the metal, you'll drop the load a lot. Some of the Midea portables re-evaporate condensate through the exhaust when there's a little water in the base, so plugging the drain and letting a bit collect can help capacity too, just watch for any water warning light.

If it still does the half-minute shut off trick on a cool morning after the hose fix, then it's a warranty call. But my money's on heat and airflow.

Something that helped me diagnose the samebehavior on a customer's garage unit last summer - I put an infrared thermometer on the hose where it exits the cabinet. Anything over about 130F on the hose body means the heat isn't moving out fast enough. Also grab a clamp meter if you have one, these small compressors should pull somewhere in the 5-7 amp range when running. If the amp draw spikes right before it clicks off, that confirms it's working against high head pressure rather than being a bad compressor. Cheap way to settle the setup vs warranty question before you tear the window seal apart again.

Does the workshop get dusty? I ask becuase my garage unit did the exact same quick-shutoff dance and it turned out the condenser side was packed with sawdust. Workshops are way dirtier than houses and those units pull air straight through the coil. Mine ran fine in the mornings too, then quit once things heated up. I blew mine out with compressed air from the wrong direction, if that makes sense, back through the fins, and the problem mostly went away. Doesn't take much gunk on the coil to overheat a small compressor. Also curious what the hose surface temp reads, that clamp meter idea above is a good call if you've got access to one.

One thing I'd add is to check wether the hot air you're blowing out the window is sneaking back in. On my garage unit the window kit never sealed as well as I assumed it did, and a good chunk of the exhaust was recirculating into the room, so the unit was cooling with pre-heated air without me realizing it. The way I caught it was putting a cheap thermometer right at the intake grille and comparing it to a reading across the room, and if that intake number is running noticeably above the actual room temperature, that's your tell. Tape up the kit gaps and see if the afternoon behavior changes before you call it a warranty issue.

Had nearly the same thing happen with a portable I ran in my old shed, and yeah it was heat related. The poster who mentioned the unit pulling room air across its condenser is right. On mine the real fix turned out to be placement. It was sitting with its back almost touching the wall, so once the room warmed up the whole side of the case was baking and it would click off just like yours. Moved it out to the middle of the floor so air could move around all the sides and the cycling basicly stopped. Totally free fix, worth trying before you touch the hose.