Refs.: GitHub #1470, #1471
latex-lab does this:
\AddToHook{begindocument} { \cs_set_eq:NN \@footnotetext \fnote_footnotetext:n }
This means that, in the usual course of things, it silently overwrites any changes made to \@footnotetext in the class or preamble. In other words, it waits until Biblatex has made whatever changes it deems necessary. Then it silently replaces the altered definition with its own.
So Biblatex happily patches footnotes, but those changes are discarded at the start of the document environment. Since Biblatex doesn't know that, it doesn't issue any warning.
However, latex-lab also does the following
\AddToHook{package/setspace/after} {\let \@footnotetext \fnote_footnotetext:n \AddToHook{fntext}[setspace]{\let\baselinestretch\setspace@singlespace}}
So if setspace is loaded, immediately afterwards, latex-lab overwrites the definition of \@footnotetext which setspace just modified and then adds code to emulate that patch.
If biblatex is loaded later in the preamble, it finds the new latex-lab footnote code, which it cannot patch, so you get a warning.
Note that the only difference in the actual footnote code used in the document is that, with setspace, code is added to emulate that package's changes to footnotes. Aside from that change, we get latex-lab's code in both cases. That is, regardless, you do not get any patches applied by biblatex. The only difference is whether you get a warning about that (with setspace loaded earlier) or not (without setspace or with setspace loaded later).
If you do not experience any actual problems as a result of this, I'd recommend just ignoring the warning for now. (I would suggest reporting it, but I'm pretty sure that will already have been done.) If you do not like the clutter, load setspace after biblatex and it will disappear.
\usepackage{biblatex} \usepackage{setspace}
But I stress again: this won't actually change the code used in the document.
If we actually need the patched functionality biblatex provides, we have to add
\toggletrue{blx@footnote}
into the code just before the footnote text is parsed and, possibly,
\togglefalse{blx@footnote}
after.
If you are loading setspace, it is probably enough to use
\makeatletter \AddToHook{begindocument}{% \AddToHook{fntext}[biblatexhack]{\toggletrue{blx@footnote}}% } \makeatother
For example, if using authoryear-icomp, which uses constrict for ibid citations, this does not work correctly without the patch, but seems OK with it:
The image on the right shows the effect of applying the patch, which I believe to be the expected behaviour.
\DocumentMetadata{ uncompress, lang=en, tagging=on, pdfstandard=UA-2, pdfstandard=A-4f, tagging-setup={math/setup={mathml-SE,mathml-AF}, extra-modules={verbatim-mo, verbatim-af}} } \documentclass[12pt]{report} \usepackage{setspace} \usepackage[style=authoryear-icomp]{biblatex} \bibliography{biblatex-examples} \makeatletter \AddToHook{begindocument}{% \AddToHook{fntext/begin}[biblatexhack]{\toggletrue{blx@footnote}}% \AddToHook{fntext/end}[biblatexhack]{\togglefalse{blx@footnote}}% } \makeatother \pagestyle{empty} \begin{document} Example\footnote{% \Autocite{aksin}.% }. Example\footnote{% \Autocite{aksin}.% }. Example\footnote{% Feetnotes are not a thing.% }. Example\footnote{% Feetnotes are not a thing.% }. Example\footnote{% Feetnotes are not a thing.% }. Example\footnote{% \Autocite{aksin}.% }. \end{document}
setspaceafterbiblatexor ignore the warning.