When summoning a custom entity with no AI. (No behaviors to move or look around) and then running the following command:

tp @e[type=ns:entity] ~ ~ ~ 0 90

the entity looks directly down for a single tick & instantly resets its look direction back to 0.

You can validate this by putting the tp command in a repeat command block, and then running:

execute @e[type=ns:entity] ~ ~ ~ setblock ^ ^ ^1 gold_block

And you will see it does place the gold_block under the entity. However, the moment the repeat command is stopped, the same command will result in the block being placed in the X,Z,-X,-Z once again.