[Lighter lines in shape]

Hi,

 

I've created a monogram, but unfortunately some of the lines in the middle of the "F" are lighter than the other lines, creating a weird, light streak. I've checked and all of the lines are in a single layer, with no hidden ones underneath and all the same pt and same colour. I have no clue how/why this happens, but it's not just in Affinity Designer, but also when I export the file as a .png or pdf etc. When I zoom in, the lines look the same. Does anyone know what I should do to stop this? I've never had this problem before!

Thanks,

Florrie