My MIL had this for many years and it did seem to become the norm as her dementia progressed. She just didn't seem aware of a partially full bladder and was resistant to suggestions that a 'just in case' wee was a good idea when e.g. setting off on a journey. Whereas before she would have gone, as most of us do, because it's sensible to do it, based on experience. With advancing dementia she no longer had memories of past experience to call on. Living in the moment, she didn't need a wee so she didn't need the toilet - end of.
The way it seemed to work was that suddenly she would become aware of a very full bladder and would make a dash for the toilet (if there was a toilet available). Toilet accidents over several years were thus more about not managing regular toileting, rather than physical lack of bladder control. Sadly that also developed in time, as was to be expected, but although she wore protective pads she was still semi-continent at the end of her life, with advanced dementia.