This is a rendering bug for the bounding box that appears when structure blocks are used to detect the size of a structure. The lines for the box appear to be rendered behind slime blocks, water, colored glass, and likely other transparent blocks.
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violine1101
Thank you for your report!
We're actually already tracking this issue in MC-167342, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
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