Tibetan books and Tibetan buildings frequently carry writing that is not in Tibetan letters at all. On the first folio of a canonical text, above a doorway, around a mandala or along the border of a painting, the script is often Lantsa, an ornamental Indic hand of the Ranjana family that Tibet adopted for exactly these purposes.
The division of labour is clear and old. Tibetan letters carry Tibetan, including translated Buddhist texts, which is nearly all of the written literature. Lantsa carries Sanskrit, and particularly the Sanskrit of mantras and of the titles under which Indian works were known. A canonical text traditionally opens with its title in Sanskrit before giving the Tibetan, and the Sanskrit is often set in Lantsa, so the reader is told at once that the work is a translation and where it came from.
The choice is aesthetic as well as functional. Lantsa is an elaborate, calligraphic script with long ornamental heads, and it reads as formal in a way that plain Tibetan letters do not, which is why it is used decoratively on surfaces nobody is expected to parse letter by letter.
Tibetan has, then, two ways of writing a Sanskrit mantra: transliterated into Tibetan letters with the reversed forms and the subjoined ha, or set in Lantsa as Sanskrit. Both are common, and the choice is one of register.