Just dropped in an 'invasion' NOT a SCOUT and was limited to 1 mech with a 55 max tonnage though that really doesn't matter since I couldn't switch my mech...
Wierd bug. I had this same kinda thing. Had to click on an Invasion call to arms to set up my dropdeck. Its worthwhile to set up your invasion deck with one sub55 tonner as your left most Invasion deck mech, so you arent messing with your deck every single time.