To the best of my ability, I observed the following behavior. I was playing Brawl and, at the beginning of my turn, my board contained (amongst other things)
-Electro, Assaulting Battery (my commander)
-Virtue of Courage
-Electrostatic Field

My hand similarly contained:
-Mountain x1
-Monstrous Rage
-Lighting Bolt

During my turn, I then:
-Played a Mountain
-Cast Monstrous Rage targeting Electro, Assaulting Battery (Spell 1). Electro adds R, Electrostatic Field pings opponent for 1, Assaulting Battery triggers and exiles Emeritus of Conflict.
-Cast Emeritus of Conflict from exile (Spell 2). No other triggers.
-Cast Lightning Bolt targetting an opponent's creature (Spell 3). Electro adds R, Electrostatic Field pings opponent for 1, Assaulting Battery triggers and exiles a card (I believe another land, but in any case, "Not a spell I can cast this turn"). Emeritus of Conflict is not prepared.

Platform: PC, installed via Steam

I believe that was all that occurred, it then moved to combat with Emeritus still unprepared. I then yielded to my opponent with Emeritus unprepared. There is a chance I played another spell (maybe Electro was in my hand/command zone before the turn began and I am misremembering?), in which case the behavior makes sense as Emeritus was my 3rd spell and would not be around to see the 3rd spell. However, I believe it still only showed 2 spells as being cast as my turn ended which (I believe) would be incorrect for that series of events.

Unfortunately, I did not have detailed logs enabled or any screen recordings at the time.