MySQL (InnoDB) deadlock - mysql

Question

We are struggling with one deadlock which is occuring multiple times a day in our production environment.
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LATEST DETECTED DEADLOCK
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2018-12-27 19:07:34 7fcef1959700
*** (1) TRANSACTION:
TRANSACTION 2125001468, ACTIVE 2 sec starting index read
mysql tables in use 1, locked 1
LOCK WAIT 3 lock struct(s), heap size 1184, 2 row lock(s), undo log entries 1
MySQL thread id 42190185, OS thread handle 0x7fcffc0b1700, query id 918842488 --- updating
UPDATE synchronization SET service_synchronized_at = NULL WHERE id = 116212
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 615 page no 288 n bits 528 index `PRIMARY` of table `app`.`synchronization` trx table locks 1 total table locks 2 trx id 2125001468 lock_mode X locks rec but not gap waiting lock hold time 2 wait time before grant 0
*** (2) TRANSACTION:
TRANSACTION 2125001355, ACTIVE 5 sec fetching rows
mysql tables in use 2, locked 2
25216 lock struct(s), heap size 3683880, 5297668 row lock(s), undo log entries 94
MySQL thread id 42189517, OS thread handle 0x7fcef1959700, query id 918842042 --- updating
UPDATE synchronization s SET s.service_synchronized_at = now() WHERE s.service_synchronized_at IS NULL AND s.user_id IN (* time consuming select to determine which users should be updated *)
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 615 page no 288 n bits 528 index `PRIMARY` of table `app`.`synchronization` trx table locks 2 total table locks 2 trx id 2125001355 lock_mode X lock hold time 3 wait time before grant 0
*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 615 page no 2313 n bits 472 index `PRIMARY` of table `app`.`synchronization` trx table locks 2 total table locks 2 trx id 2125001355 lock_mode X waiting lock hold time 0 wait time before grant 0
*** WE ROLL BACK TRANSACTION (1)
In one query I want to set service_synchronized_at = NULL. service_synchronized_at is always not null in first query before update.
In second query I add a where condition s.service_synchronized_at IS NULL thinking it would result in non-locking rows with not null values. Guess I was wrong.
The table has only primary index on id and unique constraint on user_id (and of course foreign key on user_id).
Any help is welcome.