Execute commands no longer function when a forward slash is used within the subcommand

The Bug:

Execute commands no longer function when a forward slash is used within the subcommand.

This behavior worked in 1.12.2, but was lost in 1.13.

Steps to Reproduce:

1. Launch an instance of 1.12.2 and execute the command provided below where a forward slash is used within the subcommand.

   /execute @e[type=minecraft:pig] ~ ~ ~ /say hi

2. Take note of how the command ran successfully.
3. Launch an instance of the latest version of Minecraft and execute the command provided below where a forward slash is used within the subcommand.

   /execute as @e[type=minecraft:pig] run /say hi

4. Observe if the command ran successfully or not.

Observed Behavior:

Execute commands no longer function when a forward slash is used within the subcommand.

Expected Behavior:

Execute commands would function when a forward slash is used within the subcommand.