It's unfortunately worse than that. Even with the correct spoiler tag some users will see it without it being hidden. Any service accessing lemmy, but is not lemmy. Such as kbin, doesn't support lemmy spoilers. Then you have Sync which only currently supports reddit spoilers. There currently doesn't exist a one-size fits all spoiler formatting option.

Thanks for the heads up. I used the spoiler option from the sync app. Then checked a couple other apps, each of which used the method you linked. Though for some reason, none of them actually format the text to be hidden no matter which method I used.

I used the Lemmy.world site in a web browser, and selected the spoiler option in the text editor. Don't know what is going on. When viewing the post there the contents are hidden with a spoiler drop-down.