Just happened to me today. It happened randomly, unprompted. I closed UE on Saturday, and it was normal. I opened it on Monday and got 2px thick lines. The only thing I can think of is that I was playing a UE5 game earlier and got a GPU crash, so that’s my best guess for what caused it. I updated my 3080 TI drivers to latest and updated UE from 5.4.3 → 5.4.4 and still have thick lines.
This is going to drive me crazy. I’ll probably just have to deal with adjusting the alpha for now.