My vault is small, less than 300 short notes.
The “Test sync” file did not show up for at least an hour.
Checking now, some 12 hours later, it is now available on my phone.
So it seems like the issue here is better described as “sync delay”, which may or may not be related to the invalid filenames in some notes.
I have noticed something similar with my daily notes: they’re usually not available on my phone the day I create them, but they do eventually show up.