Interestingly enough, my TSA precheck also disappeared from my boarding pass yesterday when I took my a UA flight. The mysterious thing is I just took a UA flight a couple of weeks ago and a AA flight last week. Both were with TSA pre precheck. I checked my mileage account, somehow KTN disappeared from my membership account. So I reentered the number in the airport. But the boarding pass still not showing TSA precheck. I was running out of time so I took the regular security lane. After I landed, I noticed from UA app that my flight reservation for this flight had no KTN. So I re-entered KTN to my reservation. By the time I checked in for my returned flight, the online boarding pass did not show TSA precheck. So I tried one more thing. The difference between my UA light this week and the one a couple of weeks ago is the booking channel. I booked my flight directly via UA app a couple of weeks ago. For this week's flight, I booked through Concur of my company. When I setup my Concur profile this week I did not fill in my KTN. So I went to my Concur profile and update my KTN. Then I texted TSA (275872) to confirm my precheck status. They were very responsive. They told me that my account is active and the reason I did not get the precheck in the first place was because my reservation did not have KTN. Now that I have updated my KTN, I should be good to go with precheck. My boarding pass still not refreshed with precheck. But I went to kiosk in the airport to print a copy of my boarding pass, Wala, precheck showed up!

So if your precheck disappears, you should
1) text TSA to confirm your precheck status;
2) if you status is active, check your reservation, not just your mileage account, for the KTN;
3) think about information from other booking channel might overwrite your UA information (not sure why it did not happen to AA);
4) If you have done all the above, reprint a paper boarding pass