I don't have specific information on the supply of food in Bulgaria, but I do have information on the other countries.

Sweden had gone through a near-revolt due to food shortages during Spring 1917 and had made a deal with the Entente to be able to import food, and the situation was much better in 1918.

Austria-Hungary was plagued by a lack of both locomotives and rolling stock. They lost quite a large part of both when all of it was used to run their armies to Galizia in the mobilisation, and then lost most of it as they were defeated during the battles of Galizia 1914-1915 - unfortunately, it was mostly destroyed or moved off when they recaptured the area in the Gorlice-Tarnow offensive. Thus there were severe problems moving what food that existed to the cities and towns from the farms on the countryside. This was not helped by the fact that Austria-Hungary had mobilised around 7 800 000 men (out of a population of 52 800 000, or 14,8%) during the course of the war - with a total casualty list of 7 020 000 including 1 200 000 killed, 3 620 000 wounded and 2 200 000 prisoners. Or about 90% of the mobilised men.

The army took priority in what transport was left to supply forces fighting on two fronts (as well as German supply for the Bulgarians and Ottomans passing through Austria-Hungary) and also mobilised a lot of the horses needed for farming.

Austria-Hungary had been one of the largest oil producing nations of the world before 1914, with oil wells in Galizia (that would mostly run out during the 30s, but that was not known 1914). Oil was needed for trucks and the few tractors available, but also treshing machines and other mechanised farming equipment. These sources of oil was also lost when the Russians over-ran Galizia.

So production went down, and the ability to distribute what was available also went down quite a bit.

I have also read that during the late 1800s and early 1900s, it was held that people needed a steady supply of meat-based protein to stay healthy - more than modern science would indicate. Thus grain and potatoes were fed to pigs and cattle for meat production, something which loses 90% of the calories produced. This probably made a bad situation worse.

The Central Powers were seizing food as much as they could in the occupied part of Russia, Romania and Serbia-Montenegro, which explains the food situation there.

Bulgaria never experienced invasion or the loss of a large part of their transport infrastructure. They also occupied large swaths of territory, being able to plunder Southern and Northern Dobrudja as well as Serb and parts of Greek Macedonia.

The Bulgarians also only had to support a single front (both the Germans and the Austro-Hungarians had to supply two), which probably left their transport network less strained as well.