It will depend on the individual situation. There are various exclusions and simplifications in the standard for circuits running at less than 60Vdc, but for this case they would need to be read carefully and the construction reviewed carefully to see if the exclusions can apply.
For example, often secondary circuits <60Vdc are excluded from evaluation using 8.4.2c. However if the frame of the device is metal and connected to the negative of the battery supply, open circuiting one pole of the supply (single fault condition) would create a large area at 12V (or 24V), too large for the exclusion to plausibly apply (probability of contact between the part and the patient).
If the frame is plastic and the secondary not easily accessible it could be OK. The exclusion also has a limit of 240VA so fuse size could come into it.
Applied part isolation will depend on the type of contact (electrode, plastic sensor etc), and rating (B, BF, CF).
Common sense would anyhow try to limit or isolate accessible parts. If every designer did not worry because it is only 12Vdc, there would be the potential for short circuits in the environment.
Just my guess as well: due to the lower supply voltage, supply currents are relatively high so there may be significant voltage differences (voltage drops) in the "ground" of the vehicle. For this reason it could makes sense to keep things isolated (even just functional insulation).
Note that the isolation requirements at lower voltages are quite moderate, nothing like the 4kV/8mm required for 230V mains.
