Empty error message is displayed when there is an error (except syntax error) in the targetselector in second or subsequent subcommand.

You can reproduce them by executing the following command in the chat field or in the command block.

/execute as @p as @p[type=cow] run say a

Error message is empty.

/execute as @p run tell @p[type=cow] a

Error message is empty.

/execute positioned ~ ~ ~ as @p[type=cow] run say a

Error message is empty.

/execute as @p[type=cow] as @p run say a

Error message is displayed.

/execute run execute as @p[type=cow] run say a

Error message is displayed.

/execute as @p as @p[hasitem={item=bow},hasitem={item=arrow}] run say a

Error message is empty.

/execute as @p as @f run say a

Error message is displayed.