Anyone know why unsoloing a track mutes it?

Anyone know why when I solo a track, and then unsolo it, it changes the track from solo to mute, with all the other tracks still muted?

the only way to unmute them all again is to press the solo button at the top of the arrange window, the keycommand only does what i mentioned.

Does that when another track is still soloed.

Ahh of course ! Thanks - strange as I am sure I did not have any other tracks solod, but i guess i must have.