I can confirm that the Spoiler extension is broken since the last phpBB forum software update. It causes this effect even when I disabled it. I have tried updating it to 1.3.0 and 1.3.1 beta, but this didn't help. Therefore please don't use the spoiler tag for now until it is fixed.
Edit: I have fixed it by reverting to the /vendor/s9e "Text formatter" from phpBB 3.2.5. The one in phpBB 3.2.7 seems to try to parse the spoiler tag by itself, but fails doing so.
Edit 2: The maker of the extension has replied on Github that the beta fixes the problem, but the plugin data needs to be deleted (which is only possible when the plugin is disabled). This seems to be working too.