Unwanted tabs in source mode fenced code blocks

When I’m in source mode and typing in a fenced code block, leading spaces are replaced with tabs:

Screenshot 2024-01-05 at 9.18.38 AM

… which is screwing up alignment. The result looks fine when rendered:

Screenshot 2024-01-05 at 9.21.16 AM

… but it makes editing cumbersome. Is there a way to turn this off?

Actually, source mode is irrelevant to my question. The unwanted tabs appear in any edit mode.