Usually block highlights appear as black lines, but if there is a Nether portal or water behind them, the portal and/or water become transparent where the lines are.
I would upload some images to prove it but Jira is refusing to let me upload anything because of 'missing tokens'.
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Thank you for your report!
However, we are already tracking this issue, and this report is a Duplicate of MC-187598.
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Please try to relog on Jira, or provide the screenshot via a third party image host.