Do bosses stop taking damage?

So I notice sometimes when a boss is on the verging of entering it's next phase. A crossed circle appears over its HP. I've looked up this phenomenon and although it is apparently a known thing, there is no answer as to what it means. As I'm attacking, my hits are still registering damage but because the boss's HP is so high, I can't tell if I'm still actually affecting it. Do these transitions count as momentary invincibility? Do I have to allow the transition to complete before I attacl again?

Also, I'm currently fighting Arsenal and I can't recall from previous playthroughs (and again this amazingly had no answer online) but do summons not occur during this battle? I've been fighting this thing for over an hour and the summon meter has not appeared. The last time I used a summon was against Jenova. What's going on?

So I found my own answer. I played the Reno fight on Easy mode. Got the Turk to half HP when the crossed circle popped up over his health bar. I kept swinging away. Even staggered the b-. But his health didn't move. This didn't happen when I fought Abzu. I was able to drop that shoat before he could even use Backwater Blast. This is a shait game mechanic IMO.

Yeah, now that you have confirmed it, this was something I didn't like in the battle mechanics.

I don't remember which one is Arsenal. If it's the one you fight with Barret and Aerith, you can definitely use summons. In fact a 9999 hit from Choco Mog probably saved my butt on this fight.