[uasi]

You're right. I looked up 現代仮名遣いの告示 [0] for the first time, and it says 塔（とう） is officially pronounced as "too". I had it backwards - I thought that 塔 is "tou", but due to the varying sounds of う, people could (and often preferred to) pronounce it as "too" in everyday speech.

This kind of misconception seems not uncommon. There's an FAQ on NHK's website [1] that addresses the question of whether 言う（いう） is pronounced "iu" or "yuu". The answer is "yuu", and the article make it clear that: "It's not that [iu] is used for polite/careful speech and [yuu] for casual speech - there is no such distinction."

I think native speakers learn words by hearing them and seeing them written in hiragana, before learning the underlying rules, so they know "too" is written as とう, but might not realize that とう shouldn't be pronounced as "tou" or いう as "iu". These are at least less obvious than cases like は in こんにちは never being "ha".

Personally, if I heard someone say 塔 as "tou" or 言う as "iu", I probably wouldn't count it as incorrect, nor would I even notice the phonetic difference.

[0] https://www.bunka.go.jp/kokugo_nihongo/sisaku/joho/joho/kiju...

[1] https://www.nhk.or.jp/bunken/research/kotoba/20160801_2.html

[BalinKing]

FWIW I think 言う is a different phenomenon entirely, because おう is pronounced as two vowels when it has grammatical meaning (in this case, as the verb ending), or between different words/morphemes. But my (non-native) understanding was that for nouns and such, or within the main morpheme of a verb (e.g. 葬る), “ou” is (usually) indistinguishable from “oo”.