summon no longer works with execute

Partner Rank:

1

Partner Team Reporting:

Jigarbov Productions

Verification builds:

1.16.100.52 4553528

Summary:

summon proceeding the execute command doesn't work and returns an error

Impact:
execute summon is a cornerstone of map making, this issue not only breaks hundreds of thousands of peoples content, but makes it impossible to properly test content in the R16U1 betas which means we cannot properly test our content.

Infinity Skyblock
Papercraft Adventure
Papercraft Adventure 2
Mutant Battle Arena
+++ are affected by this.

Repro Steps:

1. open a new world
2. put yourself in creative mode
3. do

   execute @a ~ ~ ~ summon cow

Expected: expect a cow to be summoned at the position of the entities targetted
Observed: the command fails, the cow is not summoned.

Screenshots/Videos attached: Yes

Regression Builds:
1.16.20

Notes: