Teleporting the initial executor affects the positions of other branches in execute command.

Here is the command that is causing the issue. Summon at least four armor stands and run the following command in chat.

/execute as @e[c=5] run tp @a ~ ~1 ~

Inexplicably, the player teleports 5 blocks up.

Adding a single positioned ~ ~ ~ subcommand fixes this. Setting the current location to the current location shouldn't have any effect, but something has changed.

/execute positioned ~ ~ ~ as @e[c=5] run tp @a ~ ~1 ~

The player teleports up only 1 block, which is the expected behaviour.