If he had dementia it may not be his first time.
He didn't seem to be suffering - totally alert and cogent from what I heard with the polling station staff. Having seen my DM fall prey to that horrible fate, it didn't chime. However I'm not expert so anything is possible.
Ultimately it's not really important. Nothing about 2019 was enough to stir the resident 1 in 3 who can't be arsed to vote, so no harm, no foul as it were.