There's no correct and incorrect here. The help page (https://support.google.com/websearch/answer/3284611) doesn't even mention which word you should use, and it of course depends on the language you use. As a German speaker, I naturally use "in", and that always worked for me (I use this a lot), except for this example.
I see that that works. In this particular case obviously better than "in". I guess I'll use this in the future, but I'm sticking with that this is a bug, not something inherently wrong with using "in".
"in" is the abbreviation for "inches" and Google will most always interpret it that way from an English language point of view. Additionally as a vernacular you usually only say "meters in feet" when your wanting to do a single unit conversion such as "how many meters are in a foot?". Google's language processing tends to be heavily slanted towards common English in which case some differences like that will never be considered how you want.
Considering you're right that that's exactly what is happening here I can't push this too hard, but I definitely think it's very common and standard English to say "what's 17.21 metres in feet?" and of course very common especially in metric countries to abbreivate metres to m. It's also a logically odd request to want to know what 17.21 million inches are in feet with no "to" or even "in" (the in having been used up here by being assumed to mean inches) to give the query meaning. That would read as "17.21 million inches feet".
Google is capable of supplying a unit conversion answer for you with the "in" construction for the query "17km in miles" for example so it understands "in" in the way many of us would expect it to but weirdly assumed that 'in' meant inches and that the query is constructed with no grammatical indication that a conversion is even being requested which is a bit of a funny leap. It's understandable that this might perhaps not have been anticipated exactly, in as much as perhaps it's not surprising that it doesn't somehow have the capacity to evaluate the likelihood of such a request over the more "common sense" interpretation but whether or not it's understandable that this mightn't have been foreseen I think we could reasonably call the resulting interpretation as undesireable for almost all human beings that might ever use this tool and it's probably fair to call it a bug in that sense.