I have my cast bars set up to left click = target and then I have keybinds to actually cast my healing spells. However, with Cell, there seems to be a weird thing where if I left click (target) someone at the same time I'm trying to cast a spell, it doesn't cast the spell. In my head, there should be an infintesmal difference between the select and cast that it casts the spell either way (either on myself or on the person I actually targeted). Even worse, it does this when I already have the person selected (if I click the frame again and cast at the same time, it doesn't do anything). I can reliably get it to do it. I never had this issue using Vuhdo, so it's probably not something that's inherent to the game. It could be another addon maybe causing problems, though -- not sure.

Here's a video showing what's happening: https://youtu.be/kFcgMqOB8t4

In this, about half the time I click cell and cast (keybind: 1) at the same time, it doesn't cast anything.

This ended up being an issue with Bartender, which was handling the keybinds. I toggled this setting on, and everything started working properly again: "Toggle Actions on Keypress instead of release".