It looks like resolvers that do strict QNAME minimisation (RFC 7816) may have this problem. I was able to reproduce this behavior on Unbound 1.6 with the “qname-minimisation-strict” flag turned on.
I suspect there may be more resolvers that run into the same problem for other reasons. If you’ve seen this issue, I’d be very curious to hear what resolver(s) you’re using - either particular software, or particular providers’ resolver IPs.