Q. [ɸt] became [ut]; [ɸt] > [ut]
In the Outline of Phonology [OP2] from the 1950s, Tolkien mentioned a late sound change of ꝑt [ɸt] > u̯t to form diphthongs with the preceding sound:
Alter this and make change of pt (Q) > ꝑt normal. Transcription pt to show bilabial pronunciation. The combination ft was avoided in Q. and Sindarin. In later (post-exilic) Quenya this ꝑt seems usually to have been pronounced u̯t with partly[?] unvoiced u̯ (cf. approach of mahtya > maitya in Ñ. Quenya). Thus lepta > leꝑta “finger” > leu̯ta (OP2: PE19/84).
Outside this one example, this phonetic development appears nowhere else and was most likely was a transient idea, since Tolkien typically represented this combination a pt in the Late Quenya of the 1950s and 60s. There is no sign of it in earlier writing, either.