/execute does not center absolute horizontal integer coordinates on block

The bug

In general 0.5 is added for absolute horizontal integer coordinates, for example /tp 4 4 4 = /tp 4.5 4.0 4.5. However, the /execute command does not appear to do this.

How to reproduce

1. Use the following command

   /execute @s 4 20 4 summon armor_stand ~ ~ ~ {NoGravity:1b}
   

   → The armor stand is summoned at 4.0, 20.0, 4.0 instead of 4.5, 20.0, 4.5