Q. [ŋ] assimilated to following [n] after [e], [a], [o]; [{eao}ŋn] > [{eao}nn]

Q. [ŋ] assimilated to following [n] after [e], [a], [o]; [{eao}ŋn] > [{eao}nn]

In cases where velar nasal ñ [ŋ] survived before a dental nasal n, it likewise became dental so that eñn, añn, oñn > enn, ann, onn. Tolkien mentioned this in both Outline of Phonetic Development [OP1] from the 1930s and the Outline of Phonology [OP2] from the 1950s:

In early PQ ñn remained as in tekna “a letter” > teñna. But in TQ pronunciation this is represented by nn, a development that may be due to actual natural pronunciation in later classical PQ, since the spelling nn is also there frequently found ... But iñn > ín; uñn > un (OP1: PE19/43).
In Quenya dissimilation, by loss of nasality, occurred ... probably in iñn, uñn > ín, ún (via iʒn, uʒn?). The only assimilation was the comparatively late assimilation of eñn, añn, oñn > enn, ann, onn ... (OP2: PE19/98).

As indicated in these quotes, the combinations iñn, uñn were an exception to this rule, with the ñ vanishing with compensatory lengthening to produce ín, ún. The OP1 quote indicates this change took place in late Parmaquesta [PQ]. Aside from the example ᴹQ. tenna “letter” given above, this sound change can also be seen in:

This sound change was mentioned in other contexts, each indicating it was a relatively recent phonetic development:

In TQ ñn > nn, hence 5{ often appears = g5. (Feanorian Alphabet, 1940s, PE22/149).
ŋn = ŋn remaining, but later > nn (phonetic notes from the 1960s, PE22/149).

References ✧ PE19/96, 98; PE22/149

Order (02800)

After 00200 [ŋ] vanished between [i], [u] and [n] with compensatory lengthening

Phonetic Rule Elements

[eŋn] > [enn] ✧ PE19/96 ( > en; before n); PE19/98 (eñn > enn)
[aŋn] > [ann] ✧ PE19/96 ( > an; before n); PE19/98 (añn > ann); PE22/149 (ŋn > nn; later)
[oŋn] > [onn] ✧ PE19/96 ( > on; before n); PE19/98 (oñn > onn)

Phonetic Rule Examples

laŋna > lanna aŋn > ann laŋna > Q. lanna ✧ PE17/65
laŋna > lanna aŋn > ann LAŊ > Q. lanna ✧ PE17/65
eŋna > enna eŋn > enn eñna > enna > Q. éna ✧ PE19/97
teŋna > tenna eŋn > enn TEÑ > Q. tenna ✧ PE19/97

ᴹQ. [ŋn] assimilated to following [n] after [e], [a], [o]; [{eao}ŋn] > [{eao}nn]

References ✧ PE19/43, 47; PE22/45

Variations

Related

Phonetic Rule Elements

[eŋn] > [enn]
[aŋn] > [ann] ✧ PE19/50 (ñn > nn; later)
[oŋn] > [onn]

Phonetic Rule Examples

teŋna > tenna eŋn > enn teknā > Q. tenna ✧ PE19/86
teŋna > tenna eŋn > enn ᴹ✶tekna > ᴹQ. teñna ✧ PE19/43