The thermal decomposition of FeCO3
Well, just like any other reactions, it depends upon the the heating temperature and the reaction environment. Contrary to what the reaction tells, the decomposition products of the reaction is actually iron(II,III) oxide, $\ce{Fe2O3}$ if it is heated in oxygen environment$\ce{^{[1]}}$ and $\ce{Fe3O4}$ if it is heated in $\ce{CO2}$ environment$\ce{^{[2]}}$.  $\ce{FeO}$ also formed during the reaction but for a small period of time. Hence it is an intermediate product.



  Figure 1 shows the thermal decomposition of iron carbonate at
  temperature at 600°C. It is shown from one DTA curve in Fig. 1 one
  exothermic peak at 495°C accompanied with a weight loss of about 32%
  (theoretical value 31.1%). This step is due to the decomposition of
  iron carbonate to $\ce{Fe2O3}$.
  
  X-ray diffraction patterns of the decomposition products of iron
  carbonate were determined for the solid calcined at 550 and 800°C
  Figure 3 (a) shows X-ray diffractogram for the sample dried at room
  temperature corresponding to iron carbonate. The diffractogram of the
  sample calcined at 550°C consists of very poorly crystallized α-Fe2O3
  phase. .Heating of ferrous carbonateat 800°C led to the formation of
  well crystalline $\ce{α-Fe2O3}$


  In air, the X-ray pattern of $\ce{FeCO3}$ was replaced by that of
  $\ce{Fe3O4}$ as the sample was heated. The change was essentially
  complete at 500°C. No further changes were noted. If, however, the
  furnace atmosphere was changed from air to $\ce{CO2}$ or helium above
  1000°C. the sample transformed slowly to $\ce{Fe3O4}$. Restoring the
  air atmosphere again resulted in the pattern of $\ce{Fe2O3}$. When
  $\ce{FeCO3}$ was decomposed in a $\ce{CO2}$ atmosphere, the first
  decomposition product observed was $\ce{Fe3O4}$.[...]the brief
  appearance of $\ce{FeO}$ was suspected in the decomposition. By
  oscillating the Geiger counter through a very small arc encompassing
  the strongest line of FeO, the presence of this oxide was indicated.
  The line appeared for only a very short time and was at no time very
  strong. In the inert atmosphere, the products of decomposition were
  $\ce{FeO}$ and $\ce{Fe3O4}$. The $\ce{Fe3O4}$ in the sample could not
  be converted to $\ce{FeO}$ even by heating to the highest temperature
  available with the furnace (about 1150°C. in helium).



So, the reaction indeed forms $\ce{FeO}$ but it gets oxidized depending on the environment the reaction is proceeded.

$$\ce{FeCO3 -&gt; FeO + CO2 }$$

$$\ce{4FeO + O2 -&gt; 2Fe2O3}$$

$$\ce{3FeO + CO2 -&gt; Fe3O4 + CO}$$ 

References


A.A. El-Bellihi, "Kinetics of Thermal Decomposition of Iron
Carbonate", Egypt. J. Chem. 53, No. 6, pp.871- 884 (2010) DOI: https://ejchem.journals.ekb.eg/article_1268_82097e00ce1c4e475f17cfec61edb6d2.pdf
H. E. KISSINGER, H. F. McMURDIE, and B. E. SIMPSON,Thermal Decomposition of Manganous and Ferrous Carbonates,DOI: https://ceramics.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1151-2916.1956.tb15639.x