Attack missed against equal AC

I've got one 13 AC enemy left in the Tower of Magic (first?) Sorak fight.

My wizard casts a Fire Bolt. Rolls 12 + 6 (spell attack bonus) - 5 (3/4 cover) = 13.

Attack roll of 13 is supposed to hit against equal 13 AC. Yet, for some reason it missed.

Save file at the start of that combat: https://drive.google.com/file/d/1tSw9XsTEdEP_3XFaEQpX9LAov7I_xX1W/view?usp=sharing

Save file made right after this turn: https://drive.google.com/file/d/1DY127pjvzJhrFSKi97vV0WDcqlsq5svx/view?usp=sharing

Game info: version 0.5.42 in Spring Hotfix Beta branch, Steam.

System info: Windows 7 (64 bit) SP1, Intel Core i5-4570, 16 GB RAM, GeForce GTX 1070, 1080p monitor.