I would always be wary of what a retailer claims about a product. Best to go straight to the source. And in this case, neither the product page
nor the product brochure
mentions anything about voltage monitoring support or any other health monitoring.
Interestingly, your M33G behaves ever-so-slightly different than mine. For kicks, after reading your post, I tried 6.40.x and 6.42.x on mine, and "/system health print" outputs a voltage line, but it's not blank...instead it prints some preposterously-high (and very inaccurate) number:
[admin@MikroTik] > /system health print
However, there is no System > Health menu in Winbox for this board, which tells me for sure that MikroTik did not intend for this model to have support for health monitoring (it's not just that it is broken and the software needs fixing).
In contrast, I can confirm for you that the hEX (750Gr3), which is based on the same SoC/platform (mmips), does have System > Health and it lists both voltage and temperature, both of which show reasonable numbers.