Faint Postives —> Negative

I’ve been using FMU, and every day I’ve been getting a very faint positive on the FRER. This morning it was the darkest. I saved the cup to test again with later and bought two other tests five hours after my FMU and now those tests are are showing negative? Have the other FRERs just been flawed? They all showed up within the time frame but this time tested with my 5 hour old FMU both tests came out negative. Pictured is the test I first took this morning.