11 month old screaming wake ups at night

Our soon to be 1 year old goes to bed very peacefully on his own (after timed Ferber sleep training, which took about 4 days). After bedtime routine, we put him in bed at 7pm awake and he happily chats with himself for a few minutes before falling asleep.

For the past 2 weeks, he has been waking up anywhere between 2am - 5am and is extremely hard to put back to sleep. We typically let him cry for about 10 minutes before going in to say "shh shh, mommy/daddy loves you, it's time for sleep". Wait another 7-10 mins, pop in. We keep it brief but these pop