setspace and quote: why is the pre-quote content spacing messed up?

I’m trying to set my main document to one-and-a-half spacing, and have single spacing for indented quotations.

I noticed a good way of doing it was suggested in How to style blockquote?, however when I try to apply that for an indented quote within a paragraph, the single spacing is applied to the preceding text:

What is the mechanism applying the spurious single space before and how to correct that?

A workaround is to do a line break between the first lipsum[66] and begin{quote}, but that is semantically ‘incorrect’ since they are supposed to belong to the same paragraph.

MWE:

documentclass[UKenglish]{article}

usepackage{babel}
usepackage[autostyle]{csquotes}
usepackage{lipsum}
usepackage{setspace}
usepackage{etoolbox}

onehalfspacing

AtBeginEnvironment{quote}{singlespacing}

begin{document}

lipsum[66]
begin{quote}
  lipsum[75]
end{quote}
lipsum[66]

end{document}

% spacingpob.tex SE 569716
documentclass[UKenglish]{article}

usepackage{babel}
usepackage[autostyle]{csquotes}
usepackage{lipsum}
usepackage{setspace}
usepackage{etoolbox}

%onehalfspacing

%AtBeginEnvironment{quote}{singlespacing}
%AtBeginEnvironment{quote}{onehalfspacingsinglespacing}

begin{document}

lipsum[66]
vspace{-0.6baselineskip}
begin{quote}
  lipsum[75]
end{quote}
lipsum[66]

end{document}