The example below is rendered using EB Garamond in Dorico. You can see the first instance of “is” doesn’t result in a ligature, but the second one does.
Is this expected? Is it related to how Dorico spaces the font, or is it something else? I can remove the ligature in the second instance by increasing the letter spacing (even to 0.01!!), but I can’t force the first instance to create a ligature by tightening it.
You and your fonts. 
I would expect the is ligature to be an OpenType “Discretionary Ligature”, which I didn’t think Dorico could use in text frames. Either way, I’d expect it to be consistent.
However, the form looks slightly different for me: there’s a loop on the join.
From a typographical point of view, I’d favour its absence.
It might have to do with the ‘s’ in ‘is’ being the final letter of the word. Historically the ‘s’ was often written in that form only at the end of a word, otherwise a ligature like ‘st’ would have been written ‘ſt’.
True, Vaughan – though while there are features on OpenType for alternate glyphs at different word positions: initial, median, ending – if I turn on “Discretionary Ligatures” in Affinity Publisher, I get the conjoined is everywhere.
I don’t get the ligatures in either position when using EB Garamond in Dorico.
If this is the reason here, and it’s such a high-level font rendering decision in Dorico’s brain, then I’m completely fine with it and glad I learned something. My objection would be if it’s random.
Thanks Ben. In my opinion (and I’m no font expert), the inconsistency in font rendering in Dorico is a problem. Initially I thought it was just because of the particular font I was using, but the number of examples of other fonts continues to rise. I’m reminded that “Th” has the same inconsistency regarding ligatures.
I was showing my wife this font last night and asking her opinion. Her first comment was, “Well, I’m no expert at that sort of thing.” And her second comment, immediately upon seeing it, was “Why is the second ‘s’ connected and the first one isn’t?”
She’s good with details, but it just goes to show these little font variances aren’t insignificant or negligible.
Have we actually nailed this down as a Dorico problem, though? I find that Minion Pro, for instance - the legit Adobe Creative Cloud version - renders differently on Word in Windows and Word on Mac.
edit: Hmm. Apple Pages has the “is” ligature off by default. If I tell it to use All Ligatures it applies the “is” ligature in both “Arise” and “is” (with the loop, as in Ben’s example), but only if the letter spacing is at 0%. Change the letter spacing in either direction and the ligature disappears.
Well, it looks like I may be wrong in this case, and Vaughan’s explanation is accurate. InDesign renders these two instances the same as Dorico does. I’d be inclined to trust InDesign as the “correct” rendering.
If that’s the case, and it’s intentional, sorry… and I learned something!
And of course you saw the feature request coming: ligature control in Paragraph Styles.
Which begs the question why don’t I get ligatures in Dorico with EB Garamond?
Yeah. That too. 
I’ve just tried this in InDesign CS6, and I get ligatures on both ‘arise’ and ‘is’, if I check ‘Discretionary Ligatures’, and none if I don’t.
I’m using the EB Garamond available from GitHub - octaviopardo/EBGaramond12
Same font, InDesign 2020, I get a ligature on the second and not on the first, regardless of wheth discretionary ligatures are turn on or off.
Grrr. Why…
Have you re-typed the ‘is’, or just pasted it?
Could this be an OS setting rather than a program setting?
I am not an expert in esoteric fonts (stopped using InDesign, et al. around 2005, although I recall from studying Attic Greek the different form of sigma used at the end of words), but the differences you and Ben are experiencing suggest something is happening outside the programs themselves.
I re-typed it.
It could be a Mac /Windows thing?
(Though last time I used Windows, it didn’t even support any ligatures! That was back on PostScript fonts, about 4000 years ago.)
My assumption would be that in addition to those glyphs being categorised as discretionary ligatures, there is some straightforward glyph substitution going on as well, i.e. there is a table that says something along the lines of “replace 'is ’ with ' '”, which could account for the difference between “arise” and “is”. The one thing I can say for sure is that Dorico itself is not doing anything specific here: it will use the liga and gsub tables in the font to produce ligatures if the font is specified to do so, but it does not provide any control over OpenType features like lining numbers, small capitals, discretionary ligatures, stylistic sets, and so on.