When applying effect processing to tracks, why does it usually convert mono to stereo, but sometimes not?   

Hi. I think it would depend if you are using mono or stereo plugins. Or if the interleave button in Console View is det to mono or stereo.
All the best.

OK thanks, guess that would be it, now I feel really dumb, LOL,  some where on some off, not sure why. Guess I assumed they were all on mono by default and totally forgot about interleave button on track, in my defense perhaps because it (and phase) doesn't appear when using narrow track console strips which I usually use, at least that's the best excuse I can think of.
 
Cheers   

We have all been there Glad it was an easy fix.
All the best.

A "Dohhhhh!" moment
 
Cheers