It is true that normally in the Schmidt reaction of ketones, amides are formed as the major product (Ref.1). However, the products from the Schmidt reaction is highly depeends on the conditions used. The other byproducts include tetrazole and urea derivatives (Ref.2).
The mechanism for tetrazole formation given by OP is acceptable but it need harsh conditions such as the presence of $\ce{NaN3}$ and $\ce{POCl3}$ as the solvent at high temperature (e.g., Ref.3). Actually, during the Schmidt reaction of ketones, the formation of terazole should be taken place even before formation of the corresponding amide. According to the mechanism (Ref.4), there should be a nitrilium ion $(\ce{R1-C#N^+-R2})$ formation, which can be considered as a N-alkylated nitrile structure:
We all knows that a nitrile reacts with hydrazoic acid $(\ce{HN3})$ in the presence of sulfuric acid to give corresponding tetrazole (Ref.5). Thus, it can be envisioned that the additional hydrazoic acid molecule may react with the nitrilium ion to produce tetrazole compound as depicted in Ref.3:
Is this a special case of reaction or are tetrazoles formed with many different ketone reactions?
It is not a special case of tetrazole formation with cyclohexanone alone. As shown in the mechanism, it is possible to prepare a tetrazole from all kind of ketones (REf.2). However, it is note worthy that Hjelte and Agback have discussed that the formation of only tetrazole with $\alpha$-tetralone even with 1:1 ketone to hydrazoic acid ratio is due to the ring size (resistance to rearrange more stable six-membered to less stable seven membered). The have isolated $45\%$ of $\alpha$-tetralone unreacted (Ref.6).
References:
- Karl Friedrich Schmidt, “Process of making derivatives of hypothetical imines including amines and their substitution products,” U.S. Patent 1,564,631, 1925.
- G. I. Koldobskii, V. A. Ostrovskii, and B. Z. Gidaspov, "Application of the Schmidt reaction for the preparation of tetrazoles (review)," Chemistry of Heterocyclic Compounds 1975, 11, 626–635 (DOI: https://doi.org/10.1007/BF00959947).
- Rajendran Sribalan, Andiappan Lavanya, Maruthan Kirubavathi, and Vediappen Padmini, "Selective synthesis of ureas and tetrazoles from amides controlled by experimental conditions using conventional and microwave irradiation," Journal of Saudi Chemical Society 2018, 22(2), 198-207 (DOI: https://doi.org/10.1016/j.jscs.2016.03.004).
- Peter A. S. Smith, “The Schmidt Reaction: Experimental Conditions and Mechanism,” J. Am. Chem. Soc. 1948, 70(1), 320–323 (DOI: https://doi.org/10.1021/ja01181a098).
- Robert M. Herbst and Charles F. Froberger, “Synthesis of Iminotetrazoline Derivatives as Trichomonacidal and Fungicidal Agents,” J. Org. Chem. 1957, 22(9), 1050–1053 (DOI: https://doi.org/10.1021/jo01360a013).
- Nils S. Hjelte and Tamara Agback, “Benzocycloalkanones in the Schmidt Reaction,” Acta Chem. Scand. 1964, 18(1), 191-194 (DOI: 10.3891/acta.chem.scand.18-0191)(PDF).
