Attempting to use flint and steel on the sides or bottom of blocks which support infinitely burning fire, such as netherrack, magma blocks and End bedrock, or on blocks which support soul fire, nothing happens. Attempting to force fire there by removing a supporting burning block will cause the fire to disappear. This means there is no way to have infinitely burning side-of-block fire, nor side-of-block soul fire.
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Comments 11

Can confirm in 21w05b.
Can confirm in 21w06a.
Can confirm in 1.16.5 and 21w08b.

Duplicate of [MC-31168]
Can confirm in 1.17.1.

Can confirm in 1.18 and 1.18.1 Pre-release 1.
Also, if you put doFireTick off you can have the sides and bottom of other types of blocks burning infinitely, it is rare that this mechanic can only be used by changing the gamerules and not by default.

Can confirm in 1.18.2.
Can confirm in 1.19.2.

Can confirm in 1.21.4.
Relates to MC-31168?