Expected behaviour
When you write a function file, and one of its line has an unfinished "execute" command (execute <any conditions> run), the file should fail and should not be able to use it.
Actual behaviour
The function file continue working, skipping the unfinished command.
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Confirmed in 1.16 Pre-release 2.

Functions only fail to load on syntax error, `/execute run` is (unfortunately) well-formed, but unable to be executed (and execution errors are silent in functions).
Related to MC-165773, but not a duplicate. It seems trying to run
/execute run
without subsequent command fails in chat and in command blocks.