This seems to work better:
$ grep -rl list-inactive-ecs?ref=v0.3.35 . | xargs sed -i '' 's/v0.3.35/v0.3.36/'
This one has been sketchy.
LC_ALL=C find . -type f -name '*.tfvars' -exec sed -i '' s/this/that/g {} +
Dry run
LC_ALL=C find . -type f -name '*.tfvars' -exec sed s/this/that/gp {} +
The -type f is just good practice; sed will complain if you give it a directory or so. -exec is preferred over xargs; you needn't bother with -print0 or anything. The {} + at the end means that find will append all results as arguments to one instance of the called command, instead of re-running it for each result. (One exception is when the maximal number of command-line arguments allowed by the OS is breached; in that case find will run more than one instance.)
This example is macOS specific because of the sed -i '' s...