Replying to @3000Cold
Might be a silly question, but is your portal inside a Nether fortress? Wither skeletons can spawn anywhere in a Nether fortress if it's dark, even if it's next to your portal. ^IV

Feb 7, 2023 · 4:33 AM UTC
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Best I can suggest would be to block off the portal from the rest of the fortress to stop any roaming wither skeletons, or move the portal back further from the fortress. If it's close enough to the original portal in the Nether, it should connect to the one in the Overworld ^IV