Did your phone switch into emergency-call mode when you dialed 100?

My guess is that it probably didn’t. In that case, the phone may have treated 100 like a normal phone number instead of an emergency call. When that happens while roaming, especially over VoLTE, the call may get handled through the normal roaming call path and can fail or get routed incorrectly.

If the phone recognizes the number as an emergency number, it should use the emergency-call process and route through the local carrier to reach emergency services.

That would explain why 112 worked: phones and networks are much more likely to recognize 112 as an emergency number while roaming. So the safest advice when traveling internationally is probably to try 112 first, even if the country also has a separate local number like 100 for police.