I believe this is a regression in the latest release, as I can’t reproduce in WPiOS 12.7.4.
iPad Pro 10.5”

iPhone Xs Max (landscape)

Note: this applies to every block type on the canvas.
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@marecar3, I wonder if this issue can be closed since I'm not seeing the issue on the current Gutenberg version. The above screenshot shows the borders displaying correctly.
@guarani I think this was fixed alongside some recent work on InnerBlocks. Let's do a thorough double-check and then close if we're not seeing the issue anywhere.
Thanks, @guarani!
Agree with @iamthomasbishop, if we are sure that it's done we can close it.
I retested on the two devices shown in the original post and this appears fixed.
| Device | Screenshot |
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| iPhone Xs Max |
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| iPad Pro 10.5" |
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| Android Emulator Foldable |
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Agree with @iamthomasbishop, if we are sure that it's done we can close it.
I've added a screenshot above from Android which also shows the left/right borders. I'm closing this now as I'm unable to reproduce.