Unable to add spaces when editing plain text fields

When editing text/text area fields (ie, the caption field on images) I am unable to add spaces. No space appears when hitting the spacebar. Double tapping will insert a period/space (“. “) but there is no other way to add spaces will typing unless you copy/paste a space.

Any idea what would cause this bug? I updated to the latest version of the theme today but the bug persists.