Evap
If you look after the time it’s likely an evap I’m afraid. I had 2 of these last month with clearblue so I just avoid them now.
Unfortunately it looks like an evaporation line. Do a red dye test, they are much more reliable Xx
If you've looked at it after taking it hours ago I wouldn't trust the result 100% you need to go off what you see within the first 5 minutes. Sorry. Maybe test again tomorrow morning and read the result straight away then x