I noticed this today while roaming in WvW, I thought I was going crazy at first as something felt off. Suddenly I couldn't engage as easily as I was the last few days. Then it hit me, my minions are being un-summoned, and going on full cooldown everytime I mount up.
This is actually proving to be quite debilitating. I hope this gets resolved soon. Until then I'll have to drop my Flesh Golem in favour of something else. I can only imagine the pain full minion players are feeling right now.