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I have encountered issues with this when changing the font type on the emulator.
Are you using the default sign-in screen, or a modified one? If this is a modified screen, check the display characteristics of the input fields. I have encountered sometimes where this was changed.
As mentioned by others, some emulators don't display the lines. If the emulator had a recent update or a change of emulator could cause this.
We can make lots of suggestions about potential causes, but without knowing the cause, any fix suggestions are a shot in the dark.