What a breakpoint icon with a question mark means ?

When studio in edit (not debugging) mode, the breakpoint appears normal (single red ball). And I can add/remove breakpoints at this file normally.

But when running in debug mode, all breakponts inside this file shows with a question-mark (?) inside of the red-ball icon.
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And I can't add new breakpoints. but I can remove the pre-added ones.

But, none of these breakpoints are functional. the program does not "stop" on any of them.

I though studio metadata got corrupted, and deleted it.. Closed studio, opened it again. but no luck. I simply can't debug it.

It's a regular dd class file.

Any clue !?