The output generated by functions that produce chat output (/say, /tellraw) and are placed in the #minecraft:load tag for functions, is not visible for the player when opening a world. The same goes for /title.
This is not the case when executing /reload, everything works properly then.
When a function tagged with #minecraft:load schedules another function with a delay of 1 to 3 ticks, its chat and title output will also not be visible for the player. When the scheduled function is scheduled with a delay of 4 ticks, it does work. However, the latter only holds when you reload the world (by quitting to title and opening the world again). I'm not sure if this is consistent across systems, because I guess that it involves the JVM warming up.
I've attached an example datapack to reproduce this issue.
Almost definitely intended. The world needs to be up and running before the player client can connect.