The fire would quickly disappear and turn back into a nether portal. The fire can even be seen even if there isn't a supporting block. Bedrock Edition does not have this issue. Sometimes a singular portal block would also appear before turning into the actual portal.
MC-196366
Lighting a nether portal would cause the fire to be visible for a brief moment.
Resolved
Won't Fix
1
(Unassigned)
Community Consensus
(Unassigned)
1.16.1, 20w30a, 1.16.2 Pre-release 1, 1.16.2 Release Candidate 1
Retrieved
Can confirm