Query Performance | Update Statistics | Ola Hallengren SPs
Depending on your version of SQL Server, maintenance plans reorganize or rebuild all of the indexes (I believe that starting w/ 2016 you can set thresholds to only process indexes with a certain level of fragmentation).

So, with Ola's scripts, you've got indexes that aren't badly fragmented enough to be rebuilt. While Ola's scripts allow you to update all statistics (depending the parameters you provide), the job may be finishing up faster than it did in the past. Therefore, some other job that makes many changes may not be done at the point when the statistics are being updated, causing enough changes to throw off the validity of the statistics.

(Aside - you noted that you were using @OnlyModifiedStatistics = 'Y'; you might see what happens if you change that to 'N', just as an experiment.)

Alternately, if (for example) you've limited the set of indexes to be considered, Ola's script will put the same limits on the statistics to be updated.

You can set up a job to update statistics using either T-SQL or maintenance plans. That could be run more often than the rebuild job.

UPDATE: I had originally presumed that Ola's script would only update statistics for indexes it had rebuilt/reorganized; it does not look like that's the case. Pulled in info I'd added to the comments.

The suggestion remains the same: have a separate job to update statistics, and run that during the week (daily, if you want to and the job doesn't take too long), and complaints should go down.