I think I know what happens here…
Most probably that Monday is another week – last one in 2018, and it makes sense. Last day of 2018 can’t be the first week of 2019 😉
So I would query it in a different way:
'date_query' => array(
'after' => '-' . (intval(date('N')) - 1) . 'days',
'inclusive' => true
)
What it does is takes all posts published after given date. And to compute that date we take current day and substract the current day of the week – so we get last Monday…