But every one I checked did. That was about 50 out of 289 photos, including some with jpeg, png, psd, & even pct extensions, so if there really is a bug, I strongly doubt it is directly related to filename extensions.
That is a thorough examination!
I still do not see clearly what is going on.
I simply tested with a new iPhoto Library, imported two new photos, that never had metadata assigned, changed the title in iPhoto's title field by adding a prefix to the full filename with extension. I added a third photo, where I removed the filename extension from the title field.
When I migrated this test library to Photos, only the last photo did show a title, the other two not.