Ah, thanks so much - that explains it all - it wasn't a bug after all, but a quirky (or obscure) feature.

The reason I couldn't understand why some of my songs had the tempo locked, but I couldn't reproduce it easily with a simple test case was because of the 'always change tempo to best tempo for style' setting.

By default this is unset so you'd think the tempo would be locked (which is exactly what I wanted), however on the tool tip it has a note that says if there are no chords past bar 5 then it considers the song to be blank and will change the tempo regardless!

Glad I got that sorted out - thanks for all your help.