If you use the setblock command to set soul fire on any block, the fire itself can exist. On Java, it is intentional that using the setblock command does not generate soul fire.
Steps to Reproduce:
Use the command
/setblock ~~~ blue_firewhile standing on any block
Observed Results:
Soul fire is placed.
Expected Results:
Soul fire should not be placed.
Now throws a "The block couldn't be placed" error in 1.16.0.63 - appears to be fixed.