One of the very few (two most probably total) things VRT taught me was that with MAV anything can happen. Absolutely anything and it often does so beware of preconceptions because they can lead you astray. Theoretically you are correct. Quiet countryside should be ideal however I’ve often been made worse by being in quiet countryside. Any unfamiliar environment can do it. How symptomatic one is currently feeling will obviously have a huge impact on reaction.
You don’t really give much detail of how walking in the countryside is affecting you? Makes it more difficult to hazard an educated guess as to why but basically once the vestibular system is affected any other than your home environment (the one space you spend most time and with which you are most totally familiar) can cause adaption issues. An already struggling system finds all the extra stimulation more than it can handle and something has to give.
The great outdoors is exactly that. It can prove just too complex for a struggling balance system. This often applies to people who have spent long periods confined in hospital or some other institution. I always remember one from end long term hospital back patient being extremely off balance outside and saying ‘outdoors, there’s so much of it’, ‘there’s no walks to give you any bearings to work from’. Indoors people quickly decondition.
Some of the reasons I know about and can appreciate from my own experience (no doubt there are many others) would include the fact that whilst indoors our range of vision is extremely limited. I’ve seen a figure of 10 feet quoted. Outside the horizon can be limitless. This difference affects the eyes. One reason why it is essential to get outside regularly. Staying indoors only makes the ultimate reaction worse as the eyes don’t adapt. Another reason is the increased level of light. I understand that Even on the dullest day the lux reading is at least 10 times higher outside than what the maximum indoor reading can be.
Anxiety is another possibility. That too can have a huge effect and incredible as it might seem sometimes I think we can suffer that without being aware.