This has happened few times actually, not anymore thankfully. If you are the person who signs off the release so decision is on you & and in my experience heres how we had handled it.
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Do you escalate and have it fixed?
It really depend on the nature of the bug, critical bug does not necessary need to resolved right away if its not hurting trust & reputation of the system. However we should still treat it as a quality threat for sure. -
Or do you delay the release?
If that issue is a blocker or P1 (threats the integrity of the product) - simply revert that change not the entire release. -
Plan for a hotfix after the release?
Yes, few times HotFix also gets deployed, (if that particular feature was a showstopper of the release) its also fine depending upon few factors:
- Promised item of the release
- Blocker
- Damages reputation
- Creates lack of trust in the product
- Or just allow it to pass if it is low-impact?
May be not - if we know for sure somethings breaking better to revert the change, raise it internally and make sure to get it fixed in the next release.
Given above the decision maker is you. However, if PO or EM is in position to take a final call - inform them everything, up to them how they plan it, you of course will still be the one to inform the impact.
Hope it helps! 