Syllable becomes left justified after pressing hyphen twice

These are melismatic syllables though, and a double dash resp. underscore is the way to tell MuseScore about this.

You can, however, also manually align the texts, via their properties

I already knew she is British. Her book comes up often. But there are others books.

Left-aligning melisma syllables as a general rule is practically universal in the US and Europe as well and has been for centuries (well, more than just a few decades) - this is hardly some case of Gould being out of step with common practice. There are, however, many cases of things being fudged for multiple verses where some have melisma at a particular point and others don't. So maybe those are the exceptions you are seeing in those hymnals. it's also the case that conventions differed, say, 300 years ago.

Anyhow, in cases where for whatever reason you wish to have a melisma syllable centered, just click the center icon to force it manually.

As a general comment:

It's important when writing music for others to read not to just do things because you personally think it is easier for you to read. The vast majority of people will find easiest that which is most familiar. Music that follows standard established convention will almost always be easier for most people to read than music that deviates from it unnecessarily.

So, relevant to the case at hand: if there is a specific hymnal used in a specific church that centers melismas according to a specific rule (e.g., in multiple verses situations with mixed melisma status only), and you are producing music to be read by the congregation of that specific church, then indeed, following the conventions of the hymnal they are most accustomed to makes sense.

For most cases, however, simply following standard procedure is going to be better for most readers, and that definitely means left-aligning melisma syllables in most cases.