My mistake was in a completely different place. In my ~/.config/nvim/LuaSnip directory, I had a tex.lua file which contained the following snippet:

s({ trig = '^', regTrig = false}, { fmta('_{<>}', { i(1) }) }),

I removed the extra pair of parentheses:

s({ trig = '^', regTrig = false}, fmta('_{<>}', { i(1) }) ),

and the error went away (after also fixing the trigger on the magic character ^).