this is caused by summons mysterys, the summon mystery and the mains jutsu are connected and you can not cast anything between them, however, the summon mystery itself is not classified as part of the mains jutsu, thus if you try to cast a jutsu during the summon mystery part, youl still be able to cast it and will not be met with the warning of you cant do that at this time, this causes an issue of wasting jutsus because of how the game treats a summon mystery, i have included a link to a youtube video where this occurs below

Youtube - interrupting position 3 with 4