My wife and I had a similar issue last month (same record, international outbound) but unlike the OP we fly United a lot (lifetime Gold here). PreCheck did not show on either of our mobile boarding passes, but when asking the two reps at the United Club about it they couldn’t see the issue in our record, and when they printed paper boarding passes as an experiment,,,,voila! TSA Pre was on both. We all left the discussion equally puzzled. I’ve never seen this.